The two angle measures are equal (which is the point of an angle bisector), so you have
y + 30 = 3y - 50
30 = 2y - 50 . . . . . . . . subtract y
80 = 2y . . . . . . . . . . . . add 50
40 = y . . . . . . . . . . . . . .divide by the coefficient of y
The value of y is 40.
