From the diagram, we can see that line l is parallel to m. Taking the angle (3x-18) on line m to line l, we will see that both 105 and (3x-18) are adjacent to each other and lies on the same striaght line l.
Since sum of angle on a straigth line is 180, hence;
105 + 3x-18 = 180
3x + 105 - 18 = 180
3x + 87 = 180
Subtract 87 from both sides;
3x+87-87 = 180-87
3x = 93
x = 93/3
x = 31
Hence the value of x is 31 degrees