LinAlg - just one more question!

sorry for starting two new threads... i don't know how to combine them!

A million years ago, an alien species built a vertical tower on a horizontal plane. When they returned they discovered that the ground had tilted so that measurements of 3 points on the ground gave coordinates of (0, 0, 0), (1, 2, 0), and (0, 1, 3). By what angle does the tower now deviate from the vertical?

So I know that you have to form a triangle thing between the two planes... but I don't know the equation of the vertical tower plane... and what to do after that!

thanks =)