(x+3y)2-2(x+3y)(x-3y)+(x-3y)2

2x+6y-(2x-6y)(x-3y)+2x-6y
2x+6y-2x^2-6xy-6xy+18y^2+2x-6y
=36y^2

(x+3y)2-2(x+3y)(x-3y)+(x-3y)2
=[(x+3y)-(x-3y)]^2
=(6y)^2
=36y^2
将x+3y看作X
x-3y看作Y,则上面的式子为：X^2-2XY+Y^2=(X-Y)^2

(x+3y)2-2(x+3y)(x-3y)+(x-3y)2
2X+6Y-2(X`2-9Y`2)+2X-6Y
4X-2X`2+18Y`2