#### Answer

$-42$.

#### Work Step by Step

If $f(x)=3x-21$ and $g(x)=3x+21$, then we can get $f(x)-g(x)=(3x-21)-(3x+21)$ by substituting.
From $(3x-21)-(3x+21)$, we can distribute -1 to the terms in the second set of parentheses. That gives $3x-21-3x-21=-42$.