Factoring an expression is the same as simplifying.
Since we are dealing with the same variable, we can remove it for now, and add it back at the end.
13-10=3
Now, we can add back the variable to get 3b
Or, you could just subtract
13b-10b=3b
Since both numbers have the same variable, you can just subtract
In this case the variable is b (a number that stands for a unknown value)
If anything is unclear please feel free to leave a comment!
Hope this helped!