$\beta = \frac{0.008}{(0.06)^2} = 2.222$
Unsystematic risk of a portfolio can be calculated as follows:
$$\sigma_\lambda-\rho_{\lambda,m}\sigma_\lambda=\sigma_\lambda(1-\rho_{\lambda,m})$$
where $\sigma_\lambda$ is the volatility of the stock $\lambda$ and $\rho_{\lambda,m}$ is the correlation between this portfolio and the market.
Written differently this is the same as:
$$\sigma_\lambda-\beta_\lambda\sigma_m$$
which means that the unsystematic risk of a portfolio is its volatility minus its beta scaled by the market volatility.
Diversifiable Risk (Unsystematic Risk)$ = Max(0.08 - 2.22*.06,0) = 0 $
If covariance were to be $0.0008$, then $\beta = 0.222$ and subsequently
Diversifiable Risk $= 0.08-.222*.06 = 0.0667$