⓵ When two angles are completely, it means that the the sum of the two angles together is 90°!
⓶ Knowing that, if your angle a is 42°, you need to subtract 42° from 90° in order to have the measure of your angle b!
⓷ You can have different equations depending on what you are looking for.
For example, you could write that :
× = 90° - angle a
Or you could writhe that :
× = (angle a + angle b) - angle a
I’m not sure what kind of equation you are looking for, but as long as your equation (so ×) equals 48° (90° - 42°), I think you should be fine!
I really hope this helps, if there’s anything let me know! ☻