Angles 4 and 5 are congruent because they form a pair of alternating interior angles. That is, these angles occur on opposite sides of the tranversal line (the line that cross both s and t, which are parallel).
Angles 3 and 4 are supplementary (their measures add to 180º).
So angle 4 also has measure 42º, which means angle 3 has measure 180º - 42º = 138º.
