The measure of the angle formed by 2 secants that have a common point outside a circle is 1/2 the difference of the measure of the further intercepted arc and the closer intercepted arc. That means that half of the the measure of arc ABC - measure of arc XYZ is the measure of angle X. Substituting the given values gives you the equation x = 1/2 (125-45)
Then you can solve.
x = 1/2 (80)
x = 40
The measure of angle X is 40 degrees