P=2l+2w
W=7+3l
So substitute into the original equation
P=2l + 2(7+3l)
78=2l +14 + 6l
-14 -14
—————-
64=8l
8=length
= 7+3(8)
=7+24
=31
Check your work
Perimeter=8+8+31+31
=78
Hope this helps
7.6m questions
10.1m answers