How do you factor this trinomial? :

The pic is blurry, but looks like 18x²-21x-15
(3x-5)(6x+3) is the factored form. 
3 and 6 are the factors of 18
-5, 3 or -3, 5 are the factors of -15
I line them up like this
3   -5
6    3
then I multiple them diagonally: 3*3=9, -5*6=-30, the sum of 9 and -30 is exactly -21, the coefficient for x. (If the sum doesn't make -21, I'd switch -5 and 3, or change to -3 and 5 to try again.) 
so the factored form is (3x-5)(6x+3)

When i did the math i got 11 cm