Consider the binormal vector B(t)=T(t)xN(t). Show that its derivative B'(t) is a scalar multiple of N(t).

So far I can prove that:
B'=T'xN+TxN'
B'=T'x(T'/|T'|)+TxN'
B'=0+TxN'
B'=TxN'

That doesn't prove the problem, any ideas?
Thanks!