another webwork question

Okay, so I've been dot producting the normal vectors/ magnitudes of the
normal vectors multiplied together and have been coming out with
-12/sqrt(609), when I plug the arc cosine of this into my calculator, like
it says to do in the book, I get 2.0786 rad and 119.1 degrees, I have no
idea why this is wrong, could you explain?

Find the angle of intersection of the plane −4x−1y−2z=−1 with the plane 3x−2y−5z=4.
Answer in radians: ________
and in degrees:  __________

**********************************************************

OK I get Cos(θ)=<-4, -1, -2>.<3, -2, -5>/(|<-4, -1, -2>|*|<3, -2, -5>|)=(-12+2+10)/whatever=0, so the planes intersect at right angles, which is π/2 radians and 90 degrees  I'm guessing bad arithmetic caused your problem.