Remark
I think you have to assume that this is a parallelogram, otherwise you are in trouble and need a more complex formula to find the area.
Step One
Set up the equation for the area.
Area = b*h
b = 2x + 9
h = 2x - 9
Area = (2x + 9)(2x - 9) = 115
Step Two
Solve the equation.
(2x + 9)(2x - 9) = 115
4x^2 + 18x - 18x - 81 = 115
4x^2 - 81 = 115 Add 81 to both sides.
4x^2 = 115 + 81
4x^2 = 196 Divide by 4
x^2 = 49 Take the square root of both sides.
x = 7
Step Three
Find the base
b = 2x + 9
b = 2*7 + 9
b = 14 + 9
b = 23 Answer
Comment
The height = 5 See if you can get this yourself. Hint use 2x - 9