To get the greatest common factor of a list of numbers, you can use a neat trick.
First, get the greatest common factor of the first two. Then, the greatest common factor of that and the third, and so on.
In this case, we need to get the greatest common factor of 24 and 40.
I'll use Euclid's algorithm (you can look that up in google, it's just a nice way to find the GCF of two numbers)
24, 40
40, 24
24, 16
16, 8
8, 0
So we get 8. Now we need to get the greatest common factor of 8 and 56:
8, 56
56, 8
8, 0
We get 8 again, and that is the greatest common factor of the three numbers.
Hope this helps :)