$\displaystyle 5\sqrt{13}=a\sqrt{3}^2 + 10^2 - 2\times{a\sqrt{3}}\times{10}\times{cos150}$

How would i work out a? how do you square $\displaystyle a\sqrt3$ and $\displaystyle 5\sqrt13$

Can someone please show me in stages how to fully expand $\displaystyle - 2\times{a\sqrt{3}}\times{10}\times{cos150}$ also as i'm not show how when surds and trigonometric functions are involved.

Many thanks guys!

F.