($\underset{a}{\rightarrow}$+$\underset{2b}{\rightarrow}$)($\underset{2a}{\rightarrow}$-$\underset{}{\rightarrow}$)
= 2$\underset{a^{2}}{\rightarrow}$ - $\underset{a}{\rightarrow}$.$\underset{b}{\rightarrow}$+$\underset{4a}{\rightarrow}$.$\underset{b}{\rightarrow}$-$\underset{2b^{2}}{\rightarrow}$
=2. $\underset{a^{2}}{\rightarrow}$ +3$\underset{a}{\rightarrow}$.$\underset{b}{\rightarrow}$ -2$\underset{b^{2}}{\rightarrow}$
=2. $4^{2}$ +3.4.5.cos135° -2.$5^{2}$
=-18-30$\sqrt{2}$