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What is an opposite number in math?
What is the opposite of 9?
What is the opposite of 12?
Yes it's true. If you want to find the opposite just add (or subtract) the number you need to reach 0, in this case +7, and then add (or subtract) +7 again... the result is the opposite. Adding (or subtracting) 2 times itself passing from negative to positive and viceversa is the same of multiplying for -1, where 1 is the neutral number in multiplication.
Yes, that's true. The absolute value of a number is its distance from $0$. With your example, we see that $-7$ is $7$ away from $0$. If we choose to think that $-7$ is to the left of $0$, then the opposite of $-7$ is $7$ to the right of $0$. That would be $+7$.
The multiplicative identity is $1$. The opposite of the multiplicative identity is $-1$. Ergo multiplying by $-1$ gives the opposite of a number.