Question 200746
f(x) =y= x^3 +9x^2 +15x -7
.
differentiate
y' = 3 x^2 +18x +15
.
factor
(3x+3)(x+5)
x=-1,,,x=-5
.
Substitute  into  first  eqn
,
y(-1) = (-1)^3 +9(-1)^2 +15(-1) -7 
y(-1) = -1 +9 -15 - 7
y(-1) = - 14
.
y(-5) = (-5)^3 +9 (-5)^2 +15 (-5) -7
y(-5) = -125 +225 -75 -7
y(-5) =  18
.