IIT JAM 2023 Question 41: Joint distributions, covariance and correlation

Let $X\sim\text{Uniform}(8,20)$ and $Z\sim\text{Uniform}(0,6)$ be independent. Let $Y=X+Z$ and $W=X-Z$. Then $\mathrm{Cov}(Y,W)$ is ________ (in integer).