Angle 3 and Angle 5 are corresponding angles. Corresponding angles lie in the same relative position. As such, they are congruent.
50x - 100 = 25x + 25
25x - 100 = 25
25x = 125
x = 5
Angle 3 = 50(5) - 100 = 250 - 100 = 150
Angle 5 = 25(5) + 25 = 125 + 25 = 150
Hope this helps!