[tex]\bf \textit{Law of Cosines}\\ \quad \\
c^2 = {{ a}}^2+{{ b}}^2-(2{{ a}}{{ b}})cos(C)\\\\
-------------------------------\\\\
\begin{cases}
c=2\\
a=4\\
b=5
\end{cases}\implies 2^2=4^2+5^2-(2ab)cos(C)
\\\\\\ (2ab)cos(C)=4^2+5^2-2^2[/tex]
and surely you know how much that is.
