Given that lines a and b are parallel, the line that crosses them, that transversal in the middle, creates the same four angles at each intersection. This means that all of the corresponding angles are congruent:
y = x
z = r
158 = t
u = s
Since t = 158 degrees, and u is supplementary to t (their angles add to 180), then we know that
158 + u = 180
Or, reorganizing,
u = 180 - 158 = 22 degrees