The 2 lines intersect, and by definition it will create equal opposite angles. Since we know that the angle created is 125 degrees, then the angle on the opposite side also must equal 125 degrees. But wait, they broke up the angle into 2 angles made up of a 64 degree angle and an X degree angle. These 2 angles must add up to 125 degrees.
x + 64 = 125
x = 125 - 64
x = 61 degrees