You want to isolate C, so the first step would to have it be positive. You would do this by adding C to both sides of the equation to cancel the C on one of the sides out like this:
[tex]p + c = r - c + c \\ p + c = r[/tex]
. Then you want to cancel out the P, so you subtract it on both sides like this:
[tex]c + p - p = r - p \\ c = r - p[/tex]
And that is your answer
