To factor, we need to find out which 2 numbers add up to -10 and multiples to be 21.
The factors of 21 are 1, 3, 7, 21.
From there, we can conclude that -3 + -7 = -10 and -3 * -7 = 21
Hence when you factor the expression, you get:
(x-3)(x-7)
