The problem asks us to evaluate the expression $(-\sqrt{21})^2$.

1. Problem Description

The problem asks us to evaluate the expression $(-\sqrt{21})^2$.

2. Solution Steps

We need to square the expression $-\sqrt{21}$.

$(-\sqrt{21})^2 = (-\sqrt{21})(-\sqrt{21})$

Recall that a negative number multiplied by a negative number results in a positive number. Thus, we have:

$(-\sqrt{21})(-\sqrt{21}) = (\sqrt{21})(\sqrt{21})$

Also, remember that $(\sqrt{a})(\sqrt{a}) = a$. Therefore:

$(\sqrt{21})(\sqrt{21}) = 21$

3. Final Answer

21