Build a website error

What am I doing wrong? Please help.
Create a CSS rule for the classes left_column and right_column (remember we already have two divs in the HTML with these classes).

Float the right column to the right and give it a width of 160px.
Set the margin-right of the left column to 170px. This will give us 10px spacing between the columns.

.left_column { margin-right: 170px; }

.right_column {
float: right;
width: 160px;
ul {
list-style: none;
margin-bottom: 15px;


In order to help you in a more expeditious manner we will need to know the lesson this is in. Please post a link to the exercise.

Above, in your CSS, is the left_column supposed to have a float property, as well?