We can keep this problem with one variable, even though there could potentially be two because of length and width. Since we can't find the width without the length, we'll make L be our variable. Here's what we know so far:
Width=L-6
Length=L
Perimeter=80
How do we find perimeter? We add length + length + width + width, or 2*length + 2*width. Wait! We already know what the length and width are -- we wrote it up there ^ ! So now we just need to set up an equation. 2(length)+2(width)=perimeter, or 2L+2(L-6)=80. Now we just solve! You should get L=23. But wait! We're not done, because while we found length, we still need to know the width. We wrote the formula for width up there ^. It's L-6. Since we know L is 23, 23-6=17.
We're done!
L=23
W=17.
To check our answer, we do the perimeter formula again. 2(23)+2(17)=80. Is this true? Yes! So we did it right!