Scaling images and videos

Hello,

i need help with the final instruction:

Finally, within the same CSS rule, set the display to block. This will instruct the images to behave as block-level elements and facilitate scaling (as opposed to their default inline behavior).

.image-container {
overflow:hidden;
max-width:100%;
height:auto;
display: block;
}

I changes the display property but i get an error message.