Math 7 grade 7. can anyone me answer these questions?

1. what is the opposite of -2?

2. when we add opposites together they will always equal = ?
-1 1 or 0

this is also known as what ?
additive inverse or opposite addition or inverse?

adding with integers with same signs.

-8 + -3 = ?

-12 + -15 = ?

-3 + -9 + -25 = ?

You are standing next to a really big circular lake. you want to measure the diameter of the lake, but you don't want to have to swim across with a measuring tape! you decide to walk around the perimeter of the lake and measure its circumference, and find that it's 400\pi\text{ m}400? m. what is the diameter dd of the lake?