7.
7 squared gives 49, and it is a prime number.
The solution for the system of equations are:

Attached is the graph of lines y=3x-2 and y=-2x+5.
P= L+W
P= Perimeter, L= Length and W=Width
Length + Width = Perimeter
So, then P(22) - W(5) = L(17)
Hope that helps.