Simplify the expression:
$-3 \sqrt{28}-3 \sqrt{112}$

Step 1 ：Simplify the expression: $-3 \sqrt{28}-3 \sqrt{112}$

Step 2 ：The square root of 28 can be simplified by finding the largest perfect square that divides 28, which is 4. So, $\sqrt{28}$ can be simplified to $2\sqrt{7}$

Step 3 ：Similarly, the square root of 112 can be simplified by finding the largest perfect square that divides 112, which is 16. So, $\sqrt{112}$ can be simplified to $4\sqrt{7}$

Step 4 ：After simplifying the square roots, we can substitute them back into the original expression and simplify it further

Step 5 ：The simplified form of the expression $-3 \sqrt{28}-3 \sqrt{112}$ is $\boxed{-18\sqrt{7}}$