(x+2y) (2x-y+3) = ….

(x+2y) (2x-y+3) = 2x^2 – xy +3x + 4xy – 2y^2 + 6y
= 2x^2 – xy + 4xy + 3x – 2y^2 + 6y
= 2x^2 +3xy – 2y^2 = 6y