Since we really only need one of each side and not for im going to divide 42/2 and use 21 for my equation.
x+(x-3)=21
2x-3=21
2x=24
(/2)
x=12
So this means the length is 12, and the width is 3 less so it’s 9.
We can check this by doing:
(12*2)+(9*2)
24+18
42