The vectors x = (-4, p, -2) and y = (-2, 3, 6) are such that $\displaystyle \arccos(\frac{4}{21}) = \theta$, where $\displaystyle \theta$ is the angle between x and y. Determine the value(s) of p.

I know that x • y must equal to 4 in this case, and the product between the magnitudes of x and y must equal to 21,

but how do you set it up so that both conditions will be true?