Angle 3 and the 100 degree angle are corresponding angles. They are both in the southeast quadrant of their four-corner angle configuration. Assuming the lines that look horizontal are parallel, this would mean angle 3 is 100 degrees. Recall that corresponding angles are congruent when we have parallel lines.
Once we know that angle 3 = 100, we will use this to find angle 4.
Angles 3 and 4 add to 180. They form a straight angle or straight line.
