First of all, let's make sure we know what a 'solution' is, so that when you
have it, you'll know it.
The 'solution' is a NUMBER. It's the number that 'p' must be, in order to
make the equation a true statement. There's only one of them.
p - 15 = 13 - 6p
Add 6p to each side: 7p - 15 = 13
Add 15 to each side: 7p = 28
Divide each side by 7 : p = 4
Was that so tough ?