A midpoint can be found on a line segment, but not a polygon like a triangle. If you're referring to the centroid, or some other such point in or around a triangle, you will need to take a few steps to do so. The centroid is my best guess to what you're referring to, and you can find it by drawing a line from the midpoint of each side of the triangle to the opposite vertex. The centroid is the point at which all three of these lines intersect, and it will always lie inside the triangle.