#### Answer

$$-19$$

#### Work Step by Step

We need to follow the order of operations according to PEMDAS for this problem. PEMDAS states that when we evaluate an expression with multiple operations, we evaluate parentheses first, then exponents and radicals, then multiplication and division, and, finally, addition and subtraction:
We evaluate parentheses first:
$$-21 - 16 - 3(-6)$$
Then, we do the multiplication:
$$-21 - 16 - (-18)$$
Now we do the subtraction from left to right, so we subtract $16$ from $-21$ first:
$$-37 - (-18)$$
We now subtract a $-18$, which means we add $18$:
$$-37 + 18$$
We now do the addition:
$$-19$$