Orthocenter of a triangle with one vertex at the origin D(0,0), E(0,7), F(6,3)
The orthocenter is the meet of the altitudes.
DE is the line x=0. That's the y axis. The altitude is the perpendicular through F(6,3), so is the horizontal line y=3.
DF is the line
6y=3x
y=(1/2)x
Perpendicular through E(0,7) is
y - 7 = (-2)(x - 0)
y = -2x + 7
That's the other altitude. Intersecting with y=3
3 = -2x + 7
-4 = -2x
x = 2
y = 3
Answer: orthocenter is (2,3)