To become horizontal angle of reflection from first mirror is 50 degree
now the ray should make same angle with M2 if its horizontal. But M2 is tilted by 25 degree. So with respect to M2 normal i=50+25=75 degree.
Now if the ray is horizontal then the angle between the emerging ray and mirror M2 is 90-25=65
so mirror has to be rotated (75-65)/2 = 5 degree counterclock wise to make emerging ray horizontal