Solve the following addition problems involving negative numbers: a) -8 + (-3) b) -12 + (-8) c) -4 + (-5) d) -7 + (-9) e) -18 + (-7) f) 6 + (-8)

Adding a negative number is equivalent to subtracting the number.

$$-8 + (-3) = -8 - 3$$

$$-8 - 3 = -11$$

The answer is $\boxed{-11}$.

$$-12 - 8 = -20$$

The answer is $\boxed{-20}$.

$$-4 - 5 = -9$$

The answer is $\boxed{-9}$.

$$-7 - 9 = -16$$

The answer is $\boxed{-16}$.

$$-18 - 7 = -25$$

The answer is $\boxed{-25}$.

$$6 - 8 = -2$$

The answer is $\boxed{-2}$.