For the first one, you would have to subtract 5 from 8 which would give you x=3!! that is your answer. For the second one, you would have to subtract 3 from -6 which would give you -a=-9, but were not done here yet, we are going to multiply this by -1 because you can't have a negative variable which will give you a=9. And last but not least, on the third one, you would have to subtract 21 from 30 which will give you c=9. There is no specific formula to do these problems other than to get rid of the "third wheel number".