If $$\sum\beta=2\left[\left(\sum_{n=1}^{N}x_i\right) \left(\sum_{n=1}^{N}y_i\right)-N\sum_{n=1}^{N}x_iy_i \right]$$ $$B={\sum\alpha\sum\epsilon-\sum\beta \sum\delta \over \sum\alpha\sum\gamma-\sum^2\beta}, $$ how can I calculate $$\sum\nolimits^2\beta? $$
The original formula come from Kasa Method.
In the page you linked, $\Sigma_\beta^2$ means $\left(\Sigma_\beta\right)^2$.
$\Sigma_\beta$ is defined in the second line of part (1).