Answer
21(SA-B)= p
Explanation
Sa=1/21p+B
First subtract B on both sides
SA-B=1/21p
And since 1/21 is a fraction the opposite of a fraction would be multiplication. And when multiple fraction you must do KCF (keep, change, flip)so it would be 21/1 which would also be 21. After multiplying 21 of both sides you will get
21(SA-B)=p