Evaluate the following algebraic expressions for the given values of the variables.

$$12$$

The numerical value is $\boxed{12}$.

2. For $x-4$ if $x=-3$:

$$-7$$

The numerical value is $\boxed{-7}$.

3. For $4x$ if $x=9$:

$$36$$

The numerical value is $\boxed{36}$.

4. For $\frac{18}{x}$ if $x=-3$:

$$-6$$

The numerical value is $\boxed{-6}$.

5. For $3x+2$ if $x=9$:

$$29$$

The numerical value is $\boxed{29}$.

6. For $7y-3$ if $y=8$:

$$53$$

The numerical value is $\boxed{53}$.

7. For $-a$ if $a=-6$:

$$6$$

The numerical value is $\boxed{6}$.

8. For $-a$ if $a=5$:

$$-5$$

The numerical value is $\boxed{-5}$.

9. For $a^2$ if $a=3$:

$$9$$

The numerical value is $\boxed{9}$.