Based on the given information, we can write the following equations:
p=w+12
p=7w
p=2w+2l
where p is the perimeter, l is the length, and w is the width.
Our goal is to find l in the third equation. But before we do this, we will need to solve for p and w in the first and second equations listed above (I am using the substitution method):
p=w+12 and p=7w
(p)=w+12
(7w)=w+12
6w=12
w=2
Now, we will use the width, w=2, to find the perimeter (it doesn't matter whether you use equation 1 or 2 here):
p=7w
p=7(2)
p=14
Now, we know both of the values of p and w, and we are ready to use them to find l using the third equation:
p=2w+2l
(14)=2(2)+2l
14=4+2l
10=2l
l=5
Therefore, the length of your rectangle is 5 inches.
I hope that helps!