Always draw a picture for stuff like this

Calling the bottom of the poster w, for width, we see that the width of the printed area is w-10-10=w-20 because of the 10cm margin on both sides

Then the side is l, for length, and because of the margin the length of the printed area is l-3-3=l-6

Since the whole poster has area of 500, so

so you can substitute that into our equations to get the length in terms of the width

Then multiply them together for the printed area expression