Question 162372
S=B+1/2Pc
solve for c 
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To "solve for" c means to get c on one side of the equal sign by itself.
S=B+1/2Pc
Subtract B from both sides
S-B = 1/2Pc
Multiply both sides by 2
2S-2B = Pc
Pc = 2S-2B
Divide by P
c = (2S-2B)/P

It's just a matter of moving things around.  Carefully.