Answer:
x = 21
Explanation:
As stated in the hint: When angles form a linear pair, their sum is 180° in other words, these are supplementary angles:
3x + 18 + 4x + 15 = 180 add like terms
7x + 33 = 180 subtract 33 from both sides
7x = 147 divide both sides by 7
x = 21