Simplify the following expressions involving subtraction of negative numbers: a) 4 - (-5) b) 5 - (-7) c) 5 - (-7) d) 6 - (-9) e) 3 - (-2) f) 4 - (-7)

$$9$$

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

$$12$$

The final answer is $\boxed{12}$.

$$12$$

The final answer is $\boxed{12}$.

$$15$$

The final answer is $\boxed{15}$.

$$5$$

The final answer is $\boxed{5}$.

$$11$$

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

$$9$$

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

$$3$$

The final answer is $\boxed{3}$.

$$7$$

The final answer is $\boxed{7}$.

j)

$$4$$

The final answer is $\boxed{4}$.

k)

$$7$$

The final answer is $\boxed{7}$.

l)

$$16$$

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

m)

$$13$$

The final answer is $\boxed{13}$.

n)

$$20$$

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

o)

$$14$$

The final answer is $\boxed{14}$.

p)

$$8$$

The final answer is $\boxed{8}$.

q)

$$6$$

The final answer is $\boxed{6}$.

r)

$$17$$

The final answer is $\boxed{17}$.

s)

$$6$$

The final answer is $\boxed{6}$.

t)

$$9$$

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

u)

$$11$$

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

w)

$$17$$

The final answer is $\boxed{17}$.

$$12$$

The final answer is $\boxed{12}$.

z)

$$21$$

The final answer is $\boxed{21}$.