the answer would be 3 I believe
The dimensions are factors of the expression
... you multiply them together to get the area
the factors are ... (x - 7) & (x + 2)
Get all the x terms on one side, and the w on the other.
Add 37x on both sides to get 56x+rx=w.
Use the distributive property backwards to get x(56+r)=w.
Divide by 56+r on both sides to get x=w/(56+r)