2x^2 - 11x - 21 = 0
We have 21*2 = 42. So, factorise 42 such that the factors have a differece of 11. Difference because the sign before last term is negative.
So, the factors of 42 which have 11 as their difference are 3*14.
So, we have,
2x^2 - 14x + 3x - 21 = 0
2x(x-7) + 3(x-7) = 0
(x-7)(2x+3) = 0
So, x = 7 and -3/2