when you subtract a number, you get the same result as when you add the opposite of that number why ?

I'll give brainliest if you answer right : )

If square root x = -7, then x= -49 true or false

The volume of gas a balloon can hold varies directly as the cube of its radius. imagine a balloon with a radius of 3 inches can hold 81 cubic inches of gas. how many cubic inches of gas must be released to reduce the radius down to 2 inches?