So,
We're going to solve the following equation:

The first thing we need to do, is to pass all the numbers (independent terms) to the other side of the equation, and let the variable p in the other side.
Notice that if we pass the numbers to the other side, they will pass with the opposite sign. Like this:

Now we're going to sum the fractions:

Now, we just multiply the denominator to the other side, and divide by the numerator:

Simplifying, we got that:

Example:
Suppose you have the equation:

We are looking for let the variable "x" alone in a side of the equation, so, to do this we could sum "1" to both sides:

The same thing happens in this problem.