I have a problem that I do not know how to start. If anyone could give me a hint on how to start it, that would be great.

Let $\displaystyle f$ be the function given by $\displaystyle f(x) = e^{-x}$, and let $\displaystyle g$ be the function given by $\displaystyle g(x) = kx$, where $\displaystyle k$ is the nonzero constant such that the graph of $\displaystyle f$ is tangent to the graph of $\displaystyle g$.

Find the x-coordinate of the point of tangency and the value of $\displaystyle k$.