Solve the following using number line.

$-9-3 = ?$

$\begin{array}{rcl}-9-3& =& ?\end{array}$

We can write -3 as +(-3) as it is an additive inverse of 3. [$\because$3 +(-3)= 0 ]

$\begin{array}{rcl} & =& -9+(\mathrm{additive} \mathrm{inverse} \mathrm{of} 3)\end{array}$

$\begin{array}{rcl} & =& -9+(-3)\end{array}$

To add a negative number, we need to move to the left on the number line.

Therefore, the answer is -12.