if the area of the rectangle below is 42 inches, find the value of x

A = LW

LW = A

(x + 3)(x - 8) = 42

x^2 - 8x + 3x - 24 = 42

x^2 - 5x - 66 = 0

(x - 11)(x + 6) = 0

x - 11 = 0    or    x + 6 = 0

x = 11         or    x = -6

The length of the side of a rectangle cannot be a negative number, so we discard the negative answer, x = -6.

Answer: x = 11