Question

use vectors to decide whether the triangle with vertices p(1,-3,-2), Q(2,0,4), R(6,-2,-5) is right-angled.

Answer 1

The direction vectors for the three sides are given by the differences in the vertices, not the vertices themselves. So consider the vectors
P−Q,P−R,Q−RP−Q,P−R,Q−Rand see whether any of these are orthogonal.