Determine all functions $f:\mathbb{Z} \rightarrow \mathbb{Z}$ which satisfy the following equations:
a)	$f(f(n))=4n+3$ $\forall$ $n \in \mathbb{Z}$;
b)	$f(f(n)-n)=2n+3$ $\forall$ $n \in \mathbb{Z}$.

1. **Injectivity of \( f \)**:
   From \( P(n) \), we have \( f(f(n)) = 4n + 3 \). Suppose \( f(a) = f(b) \). Then:
   \[
   f(f(a)) = f(f(b)) \implies 4a + 3 = 4b + 3 \implies a = b
   \]
   Therefore, \( f \) is injective.

2. **Comparing \( P(n) \) and \( Q(2n) \)**:
   From \( Q(2n) \), we have:
   \[
   f(f(2n) - 2n) = 4n + 3
   \]
   Since \( f \) is injective, we can compare this with \( P(n) \):
   \[
   f(f(n)) = 4n + 3
   \]
   This implies:
   \[
   f(2n) - 2n = f(n)
   \]
   Therefore:
   \[
   f(2n) = f(n) + 2n
   \]

3. **Finding \( f(4n) \)**:
   Using the result from the previous step:
   \[
   f(4n) = f(2(2n)) = f(2n) + 2(2n) = f(n) + 2n + 4n = f(n) + 6n
   \]

4. **Using \( P(n) \) and \( Q(4n+3) \)**:
   From \( P(n) \), we have:
   \[
   f(4n + 3) = 4f(n) + 3
   \]
   From \( Q(4n+3) \), we have:
   \[
   f(f(4n+3) - (4n+3)) = 8n + 9
   \]
   Using \( P(n) \) again:
   \[
   f(f(4n+3) - (4n+3)) = f(4f(n) + 3 - (4n + 3)) = f(4f(n) - 4n) = 8n + 9
   \]

5. **Comparing the two values for \( f(4(f(n) - n)) \)**:
   From the previous steps, we have:
   \[
   f(4(f(n) - n)) = 6f(n) - 4n + 3
   \]
   And:
   \[
   f(4(f(n) - n)) = 8n + 9
   \]
   Equating these two expressions:
   \[
   6f(n) - 4n + 3 = 8n + 9
   \]
   Solving for \( f(n) \):
   \[
   6f(n) = 12n + 6 \implies f(n) = 2n + 1
   \]

### Conclusion:
The function \( f(n) = 2n + 1 \) satisfies both given equations.

The final answer is \( \boxed{f(n) = 2n + 1} \).