They want you to factor the left side so you get \((x+4)(x+6)=(x+a)(x+b)\) where b equals 4 or 6. That's probably the answer they're looking for, but it's not correct.
The right answer is found by foiling the right side so you get \(x^2+ax+bx+ab\) and then solving for b, which gets you \((10x-ax+24)/(x+a)\). That's the correct answer. There is an infinite number of possible values for b.
