The Remainder Theorem: When a polynomial P(y) is divided by y – a the remainder is P(a)
The divisor is y – 4 so we substitute y = 4 into y3 – 9 y2 + 15 y + a: the remainder is (4)3 – 9 (4)2 + 15 (4) + a = –20 + a But we know the remainder is –7: –20 + a = –7 a = 13