Question

P•X=-10 I already know the answer but what why is P -10?

Answer 1

P-x= -10
Now to isolate x divide both sides by p
-x = -10/p
But x can not be a negative do distribute a negative 1 to both sides and your answer is :
x = 10/p

Answer 2

P•x = - 10
so in conclusion
x=10/p