angle 1 (given =75 degrees) and 4x are both interior angles between the parallel posts cut by the transversal (probably the ground).
Interior angles between parallel lines are supplementary, meaning they add up to 180 degrees, which allows us to form the following equation;
4x+75=180
solving for x
4x=180-75=105 degrees
x=105/4=26.25 degrees
(19 is a little under, you may have divided 75 by 4 instead of using supplementary angles).