Line FD is perpendicular to line EB. Line AC is perpendicular to line EB. Lines that are perpendicular to the same line are parallel. That makes lines FD and AC parallel. Angles EDF and C are corresponding angles of parallel lines cut by a transversal, so they are congruent. That makes m<C = m<EDF = 3x.
The sum of the measures of the angles of a triangle is 180 degrees.