How to solve - 10p+9p=12

We add like terms

to understand this user the distributive property in reverse like this

ab+ac=a (b+c) so we have

-10p+9p = (-10+9) p

we add the terms in the parenthsees

-10+9=9-10=-1

so we have

-1p=12

multiply both sides by - 1

p=-12