How to know the index of an element in a list

If I have a list like

[[a, b], [c, d], [e, f]]

How would I know that element a is in index 0 of the big list. I’m unsure on how to do this with a 2 dimensional array.

I tried to use index, but it not works

s = [['a', 'b'], ['c', 'd'], ['e', 'f']]

s.index('a')

Traceback (most recent call last):
  File "C:/Users/xxy/PycharmProjects/tb/test.py", line 3, in <module>
    s.index('a')
ValueError: 'a' is not in list

>Solution :

You can solve it with a loop

s = [['a', 'b'], ['c', 'd'], ['e', 'f']]
x = 'a'
find = False
for i, line in enumerate(s):
    if x in line:
        print(f'find {x} at', i, line.index(x))
        find = True
        break
if not find:
    print(f'{x} is not in list')