Answer
b could be either 16 or 8
Work Step by Step
$x^{2}$ + bx + 15
Here, the numeric factors have to be factors of 15.
Thus factors of 15 are: 1,3,5 and 15
Factors grouped such that their product is 15 $\rightarrow$ (1 $\times$ 15 and 3 $\times$ 5)
Thus $x^{2}$ + bx + 15 could be = $x^{2}$ + x + 15x + 15 = $x^{2}$+ 16x + 15 ,
or = $x^{2}$ + 3x + 5x +15 = $x^{2}$ + 8x + 15
Thus b could be 16 or 8
