For the first term of 9x2, we know 3*3 is 9 and x*x is x2. For the middle term, we need two numbers that add to zero and cancel out the middle term, but that multiply to -16.
9x2 - 0x - 16 since the middle is 0, it is cancelled 9x2 - 16 (3x - 4)(3x + 4)
CHECK: 9x2 + 12x - 12x - 16 adding +12 and -12 cancels out 9x2 - 16