The transversal cuts through the two parallelmlines a and b. At the point of intersection where you have angles 1,2,3 and 4, note that angles 3 and 4 both lie on a straight line. Therefore;
Angle 3 + Angle 4 = 180 [Angles on a straight line =180]
Angle 3 + 128 = 180
Subtract 128 from both sides of the equation
Angle 3 = 52
Next, note that angle 3 and angle 7 are corresponding angles [corresponding angles are equal]. Corresponding angles lie on the same part of two parallel lines cut by a transversal. The position of angle 3 on line a is the sam as that of angle 7 on line b.
Therefore, angle 7 = 52