Answer:
The additive inverse of 21i is -21i
Explanation:
Let b be a real or complex number, the additive inverse of b is defined as a number x, such that:

So, if we clear x from the equation, we get:
Then the additive inverse of a number b is -b. In other words, the additive inverse of a number is the one that when you add it to the previous number the result is 0.
In this case we have a complex number.
Then:
.
Where

So

.
The additive inverse of 21i is -21i