It means two do the opposite of that number.
"Other Peoples' Partners" or in more *crude* terms... "Other Peoples' Pussies/Penises" Being "down with OPP" means to be cheating on one's boyfriend/girlfriend/wife/husband
principal(in terms of math)- the amount you borrow or deposit
HL in math mean hypotenuse leg
ask your math teacher
It
sum means the answer of a math problem
No.
neither
A rhombus.
Milimeters
average