Let's evaluate: ${(-2-1)}^2$

ⓘ Parentheses are computed first: $-2-1$ = $-3$

${\left(-3\right)}^2$

ⓘ An integer power is a repeated product. The minus signs cancel out in pairs, an even power of a negative number is positive: ${\left(-3\right)}^2$ = $\left(-3\right)\times \left(-3\right)$ = $9$

▶Result:   $9$