The area as a function of b is given by
You want to find b such that a(b) = 36.
... (2/3)b^3 +6b = 36
... b^3 +9b -54 = 0 . . . . multiply by 3/2 to clear fractions
A graphing calculator shows this equation to have one real solution at b=3.
The value of b is 3.
