Show that in 3-space the distance $\displaystyle d$ from a point $\displaystyle p$ to the line $\displaystyle l$ through points $\displaystyle a$ and $\displaystyle b$ can be expressed as $\displaystyle d=\frac{|ap\times{ab}|}{|ab|}$.

I was asked this question by my girlfriend who is taking calculus three this summer. I took the class a couple semesters ago and cannot remember this stuff for the life of me. I'm refreshing now, so maybe I will get given a little more time, but for now I am pretty stuck.

Any help would be appreciated.

Thanks