Q:

Linear transformation and derivative

Let $f:\mathbb R^n \rightarrow \mathbb R^m$ be linear.  Find $D^2f(\mathbf x_0)$. 
I do not really know how to start from here.  We are not given any more details to solve the derivative.  

A:

Hint: What is the (very familiar) form of $f(x)$ if $n = m = 1$?  In that case, what's the second derivative?