6. (15%) Find the least squares solution of the following system.

$$\begin{cases} -\frac{1}{5}x1 + \frac{1}{5}x2 = 2 \\ \frac{2}{5}x1 + \frac{1}{5}x2 = 1 \\ \frac{1}{5}x1 - \frac{2}{5}x2 = 4 \end{cases}$$