He swapped the minus sign by the plus sign.
1) Considering the quadratic expression:
16x²-24x +9
Examining carefully we can note that this is a binomial (a -b)²
16x²-24x +9
(4x² - 2 * 4x*3 + 3²)
2) Hence, factoring we have that 16x²-24x +9 = (4x - 3)²
3) Therefore the mistake Jake has made is swapping the minus by the plus sign.