There's no such thing as the "length"of an angle.
But we know what you're getting at.
You have said that the angles are complementary. If two angles are
complementary, then their sum is 90 degrees, so we can take your
two angles and write:
x (one angle) plus (2x + 12) (the other one) = 90 degrees
x + 2x + 12 = 90
Subtract 12 from each side: x + 2x = 78
Add the x's on the left side: 3x = 78
Divide each side by 3 : x = 26 degrees
One angle is 'x' = 26 degrees
The other one is (2x + 12) = 52+12 = 64 degrees