Consider that angle 75° is congruent to angle (4x - 25)°.
In order to find the value of x, equal the expressions for the congruent angles, as follow:
4x - 25 = 75 add 25 both sides
4x = 75 + 25 simplify
4x = 100 divide by 4 both sides
x = 100/4
x = 25