Question

The base of a triangle in which a=58.5 in^2 height=9

Answer 1

A = 58.5     h = 9     b = ?
..............................................   

Area of a triangle 
Formula : A = 1/2 * b * h

Substitute A for 58.5 and h for 9.

A = 1 * b * h
      2
58.5 = 1 * b * 9
          2
58.5 = 9b
           2

Cross Multiply

9b = 117
b = 117
        9
b = 13

Answer 2

Area of a triangle = 1/2 x base x height
a = 58.5 
height = 9

58.5 = 1/2 x base x 9
58.5 = 4.5 x base
base = 13