Let's say I have a list with tuples in it.

Something like this:

listnum = [(18,12),(12,20)]

Is there a way I can subtract what is in the tuples and make listnum into:

listnum = [6,8]

As you can see It takes the biggest of the numbers in the tuple and subtracts it by the other.

Use list comprehension:-

>>> listnum = [(18,12),(12,20)]
>>> [(i-j) for i,j in listnum]
[6, -8]
>>> listnum = [(18,12),(12,20),(32,54),(2,43)]
>>> [(i-j) for i,j in listnum]
[6, -8, -22, -41]

And as you asked for bigger number - smaller; use abs() to calculate it.

>>> listnum = [(18,12),(12,20),(32,54),(2,43)]
>>> [abs(i-j) for i ,j in listnum]
[6, 8, 22, 41]