The perimeter of a rectangle is 42 cm. the longer side has length of 7 cm more than the shorter side. find the length of longer side

So to solve, make an equation (x being the shorter side)
2x+2(7+x)=42
So just solve from here.
2x + 14 + 2x =42
4x=28
x=7 
So the shorter side is 7, and the longer side is 14.

To find perimeter, we use this equation.
2L+2W=42
Let's say the longer side is the length.
L=W+7
Let's plug that in for L.
2(w+7)+2w=42
2w+14+2w=42
Add like terms.
4w+14=42
Subtract 14 from both sides.
4w=28
Divide.
28÷4=7=W
The shorter side is 7, which means the longer side is 14 cm.

Are you trying to figure out the volume?