How do you find the height of a triangle on a coordinate grid?

Answer 1

Pick 2 points to act as the endpoints of the base.
Find the equation of that line and write it in the form Ax + By + C = 0

If the third point is (a,b), then the distance from that point to your base, which is the height, would be
Dist = (aA + bB + C)/SQRT(A^2 + B^2)

Use the absolute value of that answer.