Its part of a bonus question on homework

(x-y)^2=x^2-2xy+y^2

Adding subtracting 4xy to right side

(x-y)^2=x^2-2xy+y^2+(4xy-4xy)

(x-y)^2=(x^2+2xy+y^2)-4xy

(x-y)^2=(x+y)^2-4xy

(x-y)^2=x^2-2xy+y^2

Adding subtracting 4xy to right side

(x-y)^2=x^2-2xy+y^2+(4xy-4xy)

(x-y)^2=(x^2+2xy+y^2)-4xy

(x-y)^2=(x+y)^2-4xy