The answer is B. (x - 9)(x + 4)
To factorise quadratic expressions, you first need to find factors of the end number, in this case 36, that have a difference of the middle number, which is 5. The two factors in this expression are 4 and 9, because 9 - 4 = 5. As for the signs, you know by the end sign that they are different, and the middle sign means that the biggest number is negative, which gives you your (x - 9)(x + 4)
I hope this helps!