Go to 'Options', select Show: 'a lot of steps' and click 'Apply':

This will add additional steps to the step by step solution that will help you understand.

Solving this equation gets us to:

-4 = 8 

A false statement (contradiction), -4 is never equal to 8.

This means no matter what value for 'p' you put you will always get a false statement '-4 = 8', or with other words, the above equation has not solutions. That is why p is element of the empty set: