I have two questions.

First: How can I align to the left all S?

Second: How to make the second line of each S be aligned with the first and not centered as it is.

```
\begin{equation}
\begin{gathered}\label{eq:suav2}
S_0^+ = \frac{240}{36}[11f_i - 18f_{i-1} + 9f_{i-2} - 2f_{i-3}]^2 + 
1040[2f_i - 5f_{i-1} + 4f_{i-2} - f_{i-3}]^2 + \\
9732[f_i - 3f_{i-1} + 3f_{i-2} - f_{i-3}]^2 \\
S_1^+ = \frac{240}{36}[f_{i-2} - 6f_{i-1} + 3f_{i} + 2f_{i+1}]^2 + 
1040[-2f_{i} + f_{i-1} + f_{i+1}]^2 + \\
9732[-3f_i + 3f_{i-1} - f_{i-2} + f_{i+1}]^2 \\
S_2^+ = \frac{240}{36}[-2f_{i-1} - 3f_{i} + 6f_{i+1} - 2f_{i+2}]^2 + 
1040[-2f_{i} + f_{i-1} + f_{i+1}]^2 + \\
9732[3f_i - f_{i-1} - 3f_{i+1} + f_{i+2}]^2 \\
S_3^+ = \frac{240}{36}[-11f_{i} + 18f_{i+1} - 9f_{i+2} + 2f_{i+3}]^2 + 
1040[2f_{i} - 5f_{i+1} + 4f_{i+2} - f_{i+3}]^2 + \\
9732[-f_i + 3f_{i+1} - 3f_{i+2} + f_{i+3}]^2 
\end{gathered}
\end{equation}

```

[![enter image description here](https://i.stack.imgur.com/PXL9Y.png)](https://i.stack.imgur.com/PXL9Y.png)