Note that m∠4 and m∠1 are vertical angles, and so they would have the same measurements (same for m∠3 & m∠2). Note that m∠1 and m∠2 are supplementary angles (going to have a total measurement of 180°)
m∠2 = 50° (given)
m∠1 + m∠2 = 180
m∠1 + 50 = 180
Isolate m∠1. Note the equal sign. What you do to one side you do to the other. Subtract 50 from both sides
m∠1 + 50 (-50) = 180 (-50)
m∠1 = 180 - 50
m∠1 = 130°
m∠1 = 130° is your answer
hope this helps