Let the base be b.
The height is b - 9.
Area of triangle = 1/2 x base x height
56 m² = 1/2 x b x (b - 9)
56 = -1/2b(b - 9)
112 = b² - 9b
b² - 9b - 112 = 0
(b + 7)(b - 16) = 0
b = - 7 (rejected, b cannot be negative) or b = 16
Answer: The base is 16 m
