I want to sort the rows of a 2D array based on the elements of the first column, in Python 3. For example, if 

```
    x = array([[ 5. ,  9. ,  2. ,  6. ],
               [ 7. , 12. ,  3.5,  8. ],
               [ 2. ,  6. ,  7. ,  9. ]])

```

then I need the sorted array to be 

```
    x = array([[ 2. ,  6. ,  7. ,  9. ],
               [ 5. ,  9. ,  2. ,  6. ],
               [ 7. , 12. ,  3.5,  8. ]])

```

How can I do that? A similar question was asked and answered [here](https://stackoverflow.com/questions/31754342/sorting-a-2d-array-based-on-the-first-column), but it does not work for me.