(-7w^2 - 2w - 1) - (-5w^2 + 3w - 2)
can someone (thoroughly) explain to me how to do this? you

(-7w^2 - 2w - 1) - (-5w^2 + 3w - 2)
Take out of parenthesis according to the symbol outside of them. In this case it is a negative so everything in the parenthesis will be opposite of what it is.
-7w^2 - 2w -1 + 5w^2 - 3w +2 
Combine like terms. 
-2^2 - 5w + 1 
That is your official answer. 
You cannot continue to simplify because there are no like terms to simplify and it would just not make sense to further simplify. Hope this helps. :)  

3were boys and 5 were girls