Answer:
x = 18
Step-by-step explanation:
The sum of two interior angles im a triangle is equal to one exterior angle that is supplementary to the third interior angle:
x + 18 + x + 12 = x + 48 add like terms
2x + 30 = x + 48 transfer like terms to the same side of the equation
2x - x = 48 - 30
x = 18
