The abscissa of m and F2 is the same, both are C, and they are brought into the ellipse. Combining the relationship between a, b and c, a 2-b 2 = c 2, the coordinates of m can be obtained.
(The slope of MN is 3/4, f 1 On the straight line MN, you can get what you want by finding the slope of the straight line with m and F 1. )