First, let's use the following formula about the margin error.
![ME=\frac{z\cdot\sigma}{\sqrt[]{n}}](https://img.qammunity.org/2023/formulas/mathematics/college/3n6tub0wrc3rmxcxno904r6bg9kya9gkzv.png)
But, we use n to find the degrees of freedom.

Using a calculator, we get a critical value t-score of 3.5.
Now we can find the margin error.
![ME=\frac{3.5\cdot4}{\sqrt[]{8}}\approx4.95](https://img.qammunity.org/2023/formulas/mathematics/college/j49na2x7r0yblhspsagj7crkmtsviyo5un.png)
Then, we form the confidence interval using the mean.
[tex]\begin{gathered} 13.2-4.95
Therefore, the confidence interval is 8.25 < m < 18.15.