Note: This is the answer if you are talking about a triangle because you didn't specify that.
A=30°
I'm going to assume this is a triangle so if I'm wrong tell me in the comments. If it is a triangle all the angles should add up to 180 degrees. We already know that one is 90 so the other two angles have to add up to 90 (since 90+90=180). C is twice as much as A. So 1/3 of 90 is A and 2/3 of 90 is C. <em>A would be 30°</em>. In summary A=30° B=90° C=60°. Hopefully I understood your question correctly and this helped you. Also sorry it took a while to answer.
