algebra

Presumably this is homework which we don't "do" but give hints.

Steps:
1. Solve the simultaneous equations
y1 =mx1 + c and y2 =mx2 + c
for m and c, using the points (x1,y1) and (x2, y2)
thereby establishing the equation of the line through the two points.

2. Show that the third point (x3,y3) satisfies the equation.