## Using Stoke's Theorem

I know how to apply Stokes theorem in a question - but I don't know how to use it here

'Use Stokes' theorem to show that
$\oint_C\phi d \mathbf{r} =-\iint_S\nabla\phi\times d \mathbf{S}$
where C is a suitably oriented simple closed curve bounding the smooth
surface S'