Hi again

I am looking for a unit vector that is perpendicular to the following vectors

$\displaystyle a= 3i-j+4k$

$\displaystyle b=-3i-2j+2k$

So I know that find a perpendicular vector that$\displaystyle a.b=0$So I get $\displaystyle (3,-1,4).(-3,-2,2)= (-9,2,8)$ but this isn't one of the options. I know that multiples of it can be also perpendicular, can someone point out where I'm going wrong?

Thanks