There are two ways we can solve this problem: using vertical angles or supplementary angles/a linear pair.
Vertical Angles:
---Vertical angles are those that are directly opposite each other within a pair of intersecting lines.
---Vertical angles are congruent.
(6x - 21) and 105 are vertical angles, and thus congruent.
6x - 21 = 105
6x = 126
x = 21
Supplementary Angles/Linear Pair:
---Supplementary angles form a line and thus add up to 180.
---A linear pair is two adjacent angles that form a line, 180 degrees.
(6x - 21) and 75 are supplementary angles/a linear pair, so their sum is equal to 180.
6x - 21 + 75 = 180
6x + 54 = 180
6x = 126
x = 21
Answer: x = 21
Hope this helps!