Find the length of each edge of the original cube.

If the length of each edge of a cube is increased by 3cm, its volume is increased by 387 cu.m. Find the length of each edge of the original cube.
a 5
b 6
c 7
d 8

let x be the edge of the original cube
(x+3)^3 - 387 = x^3
x^3 +9x^2+27x+27 -387 = x^3
9x^2+27x+27 -387 = 0
9x^2+27x-360 = 0
9(x^2+3x-40) = 0
9(x-5)(x+8) = 0
x = 5 cm