Find $b$ Such That the Average Rate of Change of the Function

Multiple Choice

Find $b$ such that the average rate of change of the function $f(x)$ on the interval is equal to the given value.
- $f(x) =x^{2}+9 x-34,[-7, b]$ ; average rate of change: -40

A) $b=28$
B) $b=-28$
C) $b=42$
D) $b=-42$