\documentclass{article}
\begin{document}
\[
u(t) = e^{-\frac{\gamma}{2M}} \cdot
	\left(
		Acos\left(
			\sqrt((\frac{\gamma}{2M})^2 - k/m)
			\cdot t \right)
		+ Bsin\left(
			\sqrt((\frac{\gamma}{2M})^2 - k/m)
			\cdot t \right)
	\right)
\]
\end{document}