Aloha :)
Willkommen in der Mathelounge... \o/
$$\overbrace{\underbrace{(x+y)\cdot(x-y)}_{=x^2-y^2}}^{\text{3-te bin. Formel}}+y^2=(x^2-y^2)+y^2=x^2$$
$$\overbrace{\underbrace{(3u+3v)^2}_{=(3u)^2+2\cdot3u\cdot3v+(3v)^2}}^{\text{1-te bin. Formel}}-\overbrace{\underbrace{(3u+v)\cdot(3u-v)}_{=(3u)^2-v^2}}^{\text{3-te bin. Formel}}$$$$=\left(\;(3u)^2+2\cdot3u\cdot3v+(3v)^2\;\right)-\left(\;(3u)^2-v^2\;\right)$$$$=(9u^2+18uv+9v^2)-(9u^2-v^2)=18uv+10v^2$$