Problem 1
The two angles 6x and 30 are vertical angles, so they are congruent or the same in measure.
6x = 30
x = 30/6
x = 5 is the answer
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Problem 2
The two angles form a straight angle which is 180 degrees. We consider them a linear pair (since they are adjacent and supplementary).
So,
5x+(3x+12) = 180
8x+12 = 180
8x = 180-12
8x = 168
x = 168/8
x = 21 is the answer