Linear Models and Multivariate practice

Consider the simple linear regression model $Y_{i} = \beta{}x_{i} + \varepsilon_{i}, i = 1, \dots, n$, where $E(\varepsilon_{i}) = 0, Cov(\varepsilon_{i}, \varepsilon_{k}) = 0$ for i ≠ k and $Var(\varepsilon_{i}) = x_{i}^{2}\sigma^{2}$. The best linear unbiased estimator of $\beta$ is