In the first triangle on the left - Example 1 is the same -- draw the line identical to it
Middle triangle is the same as Example 2 -- the left side is the altitude and the base is the bottom segment
The third triangle altitude is drawn from the bottom right point to the midpoint of the longest side -- make sure your altitude is perpendicular to the longest side so you have a right angle at the intersect.
Formula is: 1/2 (base) x (height) (height is the same as altitude)