Substituting 10 for p and 3 for b, we know that 10=2a+3r. Since we don't know r or a, we can't get an exact value for either one. We can solve for a, though, like this:
Subtract 3r from both sides to get 10-3r=2a.
Divide both sides by 2 to get (10-3r)/2 =a
Hope that helps!