Respuesta :

Answer

D, 12m

Step-by-step explanation:

MathPhys

Answer:

12 m

Step-by-step explanation:

Area of a triangle is:

A = ½ bh

Given that h = b - 5 and A = 42:

42 = ½ b (b - 5)

84 = b² - 5b

0 = b² - 5b - 84

0 = (b + 7) (b - 12)

b = -7, 12

The length of the base can't be negative, so b = 12 m.