So instead of finding the 2 numbers that multiply to get 32 and add to get 28 you need to multiply 32 by the 3 from 3x^2.
32(3)=96
4(24)=96
24+4=28
Then you replace 28x with the 2 new numbers
3x^2+24x+4x+32
3x(x+8)4(x+8)
Then group the 3x and 4
(3x+4)(x+8)