Find the coefficient  of x2 in (3x + y)3

27 27x 27y 6x

27y

(3x + y)3 = 27x3 + y3 + 9xy (3x + y)                = 27x3 + y3 + 27yx2 + 6xy2 ⇒ Coefficient  of x2 = 27y