Hi

I need help on the following question:

Find a unit vector perpendicular to the plane of the two vectors (2,-1,1) and (3,4,-1). What is the sine of the angle between these two vectors?

I have found that the unit vector perpendicular to the plane is:

$\displaystyle \frac{-3i+5j+11k}{\sqrt{155}}$

However how would i work out the angle. i used the $\displaystyle a x b = absin\theta $ equation but cannot get the right answer.

P.S