What does 'supplementary' mean ?
It means the two angles add up to 180 degrees.
So ...
(2x + 12) + (x) = 180
Eliminate parentheses: 2x + 12 + x = 180
Add up the 'x' terms: 3x + 12 = 180
Subtract 12 from each side: 3x = 168
Divide each side by 3 : x = 56 degrees