Negative in Integral Domain is Unique

Theorem
Let $\left({D, +, \times}\right)$ be an integral domain.

Let $a \in R$.

Then the negative $-a$ of $a$ is unique.

Proof
From the definition of an integral domain, $\left({D, +, \times}\right)$ is a ring.

The result follows from Ring Negatives are Unique.