Hello there!
P is an unknown value. They are asking you to find what the value P could be.
Let me show you
13 + 3p = 7
Subtract 13 from both sides
13 - 13 + 3p = 7 -13
3p = -6
Divide both sides by 3 to find p
3p/3 = -6/3
p = -2
I hope I helped, please let me know if you have addition questions :)