In the given figure, line a and b are paralle and c is the transversal
Since, angle 72 and the angle (3x+12) are the pair of supplementary angles
and the sum of supplementary angles is 180
So,
72 + 3x+12=180
Simplify and solve for x
3x+84=180
3x=180-84
3x=96
x=96/3
x=32
Answer : x = 32