How do I solve this initial value heat equation problem, using the heat equation kernal?

Question

\begin{equation} \frac{\partial \theta}{\partial t}=k\frac{\partial^2 \theta}{\partial x^2}\\ \theta(x,0)=\exp[-x] \end{equation} I know the general solution is \begin{equation} \theta(x,t)=\dfrac{1}{2\sqrt{\pi kt}}\int_{-\infty}^{\infty}\exp[{\dfrac{-(x-y)^2}{4kt}}]\exp[-y] dy \end{equation} I don't understand what to do next. Please help.