The area of a triangle is A = b * h / 2 where A, b, and h are the area, base and height respectively. We know that A, b, and h equal 47, x, and x respectively so we can write:
47 = x * x / 2
47 = x² / 2
94 = x²
x = ±√94
In context, x can't be negative so the answer is x = √94 = 9.7.