1. **Problem:** Determine the formula for the $n$-th term of the arithmetic sequence 3, 7, 11, 15, ... 2. **Formula:** The $n$-th term of an arithmetic sequence is given by: $$a_n = a_1 + (n-1)d$$ where $a_1$ is the first term and $d$ is the common difference. 3. **Find $a_1$ and $d$:** - First term $a_1 = 3$ - Common difference $d = 7 - 3 = 4$ 4. **Write the formula:** $$a_n = 3 + (n-1) \times 4$$ 5. **Simplify:** $$a_n = 3 + 4n - 4 = 4n - 1$$ **Final answer:** $$a_n = 4n - 1$$