The area of the triangle could be determined by multiplying the length and the width. Since, we are given the area and the length, to find the width, you divide the area by the length:
w = A ÷ l
w = (x2 - 2x - 15) ÷ (x+3)
What do you have to multiply to (x+3) to yield x2 - 2x - 15? That would be x - 5. Since the sum of -5 and 3 = -2 (middle term) and the their product is -15 (last term).
Hence, the width is (x-5).