It's a simple linear equation in 'P'.
There is only one number that 'P' can be that would make the equation
a true statement. The number is called the"solution" of the equation.
Here's one way to find it:
You said that 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