We have the following configuration where the triangle OAB is isosceles and the angles lambda = 20 °, alpha = 60 ° and beta = 50 ° are given.
It is a question of finding the value of gamma.
Make a correct drawing
The drawing provided is grossly wrong. By applying yourself with a ruler and a protractor, you will find 80 °, unless that is a very close number of course. How to know ? There is only one solution for that: you have to prove it!