The area of a rectangular deck can be modeled by f(x) = 2x3 + 30x2 – 6x – 90. if x + 15 represents the width, which expression represents the length?

Area of a rectangle =  length * width

f(x) = 2x³ + 30x² – 6x – 90
f(x) = 2(x³ + 15x² – 3x – 45)
f(x) = 2(x²(x + 15) – 3(x+15))
f(x) = 2(x²-3)(x+15)

If width = x+15, then 
length = 2(x²-3) = 2x²-6