Answer:
50 degrees. Complementary angles are any two angles that add up to 90 degrees. In this case, if the complement is 30 less than its bigger angle, and the bigger angle is twice the size of its complementary angle, then that would mean you take the unknown angle as x.(WORK SHOWN BELOW)
Explanation:

Then regroup

Work with like terms. For example, a whole's like term is another whole.

If 3x is the same as 150, then 150 divided by 3 is also x. We know this because almost any correct equation with some form of a constant or variable can be flipped into the opposite of itself.

And lastly, 150 divided by 3 is 50.
Hope this helps!