If triangles ABC and DEF are congruent, it means that their corresponding sides are congruent as well. So this means angle A corresponds to angle D, B with E and C with F. Now, in order to find the value of x in angle F, create an equation. Since angle C corresponds with angle F, angle C= angle F. So you could say, 50 degrees= 2x-8 degrees, or simply 50=2x-8. Now solve the equation, and you will get 29 as x.
Hope the explanation helped!