use the equation p=2l+2w and sub everything in to get 280=2l+2(l+12).
distribute the 2 for 280=2l+2l+24.
add/subtract like terms over, leaving you with 256=4l.
divide the 4 from 256, resulting in l=64.
since w=l+12, add 12 to 64 to find that w=76.
basically, l=64 and w=76.
hope this helps!