Torsion of a curve question

[kankerfist]

I have come across the following question while studying for my calc 3 exam. For each t at which
$\displaystyle \vec T'(t)$ ≠ $\displaystyle \vec 0$
there is a number $\displaystyle \tau (t)$ such that
$\displaystyle \vec B'(t) = \tau (t)\vec N(t)$
where
$\displaystyle \vec N(t),\vec B'(t),\tau (t)$
are the normal vector, binormal vector, and torsion, respetively. Show that the magnitude of the derivative of the binormal vector,$\displaystyle ||\vec B'||$, is equal to the absolute value of the torsion and that a positive value of torsion corresponds to the curve twisting toward $\displaystyle \vec N$. I am not sure where to begin on this one, so any pointers to help me get started would be really appreciated.