For instance, a function would take a set of *n* points like: { (x0, y0), (x1, y1), … (xn-1, yn-1) }.
The function would then return the sum of all these x coordinates: x0 + x1 + x2 + … + xn-1.

So, here is what I have tried to do so far,

```py
def sum_x(l):
    sum(l,key=lambda x:[0])

```