word problem using polynomials

hi i need help with this question, i got some of it...

The volume of a cardboard box is $\displaystyle x^3 - 8x^2 + 17x - 10$ Factor this polynomial.

**i got (x-1)(x-5)(x-2)**

if these factors represent the length, width, and height, find the demensions of a box that has a volume of $\displaystyle 20cm^3$

**i got ...**

**$\displaystyle x^3 - 8x^2 + 17x - 10 = 20 $**

**$\displaystyle x^3 - 8x^2 + 17x - 30 = 0 $**

**i then found a factor of (x-6), and after putting that through senthetic division i was left with $\displaystyle x^2 - 2x + 5 $, but this dosent really factor, so i think i've done something wrong...help please**