You can put this solution on YOUR website! Let the angle be A, then its supplement would be 180-A. The problem description states that the ratio of these two is 3:7, so... Cross multiply. Simplify. Add 3A to both sides. Divide both sides by 10. degrees.
Now you are to show the ratio of the angle A to its complement.
The complement of angle A is 90-A, so the ratio is: =