f(x)=-8+8x^3

y = -8 + 8x^3

y + 8 = 8x^3

[ y + 8 ] / 8 = x^3       take the cube root

 ∛ [ y + 8 ] / 2 =  x         swap x and y

 ∛ [ x + 8 ] / 2 = y = the inverse