
9x²  6x  24
= 3(3x²  2x  8) <— factored 3 out first
= 3(3x²  6x + 4x  8) <— split 2x into two terms that multiply to get 24 and add to get 2...factoring by grouping
= 3[3x(x  2) + 4(x  2)] <— factored 3x out of first two terms and 4 out of last two terms
= 3(3x + 4)(x  2) <— factored out the common factor, (x  2)
 Proof
