A and B are vectors in the direction of the two lines.

A.B = <-2,4,4> <8,-1,4> != 0 therefore the lines are >not< perpendicular.

Note:

A x B gives a vector perpendicular to A and B. I don't know why you've found it - it's irrelevant and has in fact completely mislead you into making a mistake.

Because A x B is perpendicular to A and B it's no surprise that you get

<-2,4,4><20,40,-30> = 0

and

<8,-1,4><20,40,-30> = 0.

These results have absolutely nothing to do with the question.