The line n intersects line m and at the point of intersection two angles are formed, which are 3x and x.
Please note that angles on a straight line equals 180 degrees. That means angle 3x and angle x both sum up to 180. This can be expressed as;
3x + x = 180
4x = 180
Divide both sides of the equation by 4 (to eliminate the 4 on the left hand side and isolate the x)
x = 45
