I want take out the numbers then add it together for example from this:

'a12bcd3'

my answer should be 6

how do I extract the numbers and add them together?

Python strings are sequences; looping over them gives you individual characters. If any character is a digit (test with str.isdigit()), turn it into an integer using int() and sum() those:

total = sum(int(c) for c in inputstring if c.isdigit())

Demo:

>>> inputstring = 'a12bcd3'
>>> sum(int(c) for c in inputstring if c.isdigit())
6