The question said that the points of A, B and C were (-2,-3), (6,1) and (k,3) respectively, and that AB is perpendicular to BC.

It asked to work out the gradient of AB:

y2-y1 / x2-x1 = 4/8 --> 1/2.

I think that might be right but then it says to prove that the value of k equals 5. How could I do that?