I don't understand how this works... the equation for a cross product is:

A x B = A B sin()

so like (this is a mistake, right?)

A B C

x |3| |0| |0|

y |1| * |3| = |3|

z |4| |2| |8|

so now I've got vector C, but it won't be perpendicular to AB and multiplying C by the sine of the angle between A and B will just increase the magnitude of C.

This must look very obvious to most people here, how do you do it right? I haven't taken calculous yet.