partial differential initial value problem

Hi! I am currently trying to find a formula which solves the problem:

$\displaystyle u_t-\triangle u+cu=f $ on $\displaystyle \mathbb{R}^n$ x $\displaystyle (0,\infty)$ and

$\displaystyle u(x,0)=g(x)$ for $\displaystyle x \in \mathbb{R}^n$, where $\displaystyle c \in \mathbb{R}$

I do not have a clue how to do this. How can i handle the $\displaystyle cu$? Does anybody have ideas for a solution?

EDIT: $\displaystyle \triangle u$ meaning the laplacian of u!