As mentioned in the exercise,
The slope is a measure of how steep the line is, while the intercept is a measure of where the line hits the yaxis.
The equation of a line is
y = mx + b
// m: Slope
// b: yintercept
In the exercise, we have been provided initial values of m = 8
and b = 40
and have been asked in Step 3 to experiment with these value so that the line matches the revenue data. So, our initial equation of line is:
y = 8x + 40
Look at three situations:
 Situation 1: Keep
m
(slope) fixed, but change b
(yintercept)
If you don’t change slope, then the steepness of the line remains unchanged. Changing the yintercept just moves the line up and down while preserving the steepness. For example,
y = 8x + 60
will shift the line up so that it intercepts the yaxis at a value of 60
, but steepness remains unchanged.
y = 8x + 10
will shift the line down so that it intercepts the yaxis at a value of 10
, but steepness remains unchanged.
y = 8x  30
will shift the line down so that it intercepts the yaxis at a value of 30
, but steepness remains unchanged.
y = 8x + 0
will shift the line so that it intercepts the yaxis at a value of 0
i.e. it passes through the origin of the axes, but steepness remains unchanged.
 Situation 2: Change
m
(slope), but keep b
(yintercept) fixed
If yintercept is fixed, then we aren’t moving the line up and down. Instead, changing the slope means the steepness of the line is changed. For example,
y = 25x + 40
will intersect the yaxis at a value of 40
, but it will rise sharply as we go from left to right along the xaxis (steeper line).
y = 2x + 40
will intersect the yaxis at a value of 40
, but it will rise less sharply as we go from left to right along the xaxis (less steeper line).
y = 3x + 40
will intersect the yaxis at a value of 40
, but it will slant downwards as we go from left to right along the xaxis.
y = 0x + 40
will intersect the yaxis at a value of 40
, and since slope/steepness is 0
, so it will be a horizontal line.
 Situation 3: Change
m
(slope), and change b
(yintercept)
This will change both the steepness of the line and the point where yaxis is intersected. Both the shape (steepness) of the line as well as the yintercept (moving line up and down) are being changed. For Example,
y = 3x + 15
will intersect the yaxis at a value of 15
and the slope of the line will be 3
In the exercise, you are expected to experiment with values of m
and b
(i.e. by changing the steepness of the line and moving it up and down) so that the line matches the revenue data.
A visual experiment will be much more instructive than what I wrote. So, I suggest you do the following:

Go to an online graphing calculator (https://www.desmos.com/calculator)

Enter an equation such as y = 2x + 11
. A line will appear. You can zoom in and out to get a clearer picture.

Now experiment with different values of m
and b
. What happens if you type in y = 2x + 30
or y = 2x  5
or y = 4x + 11
or y = 16x + 11
or ...
? See how different values of m
change the steepness of the line, but doesn’t change the yintercept. Observe how changing b
moves the line up and down, but doesn’t affect steepness. When both m
and b
are changed, then both changes happen together.