The factors of 139 are 1, 139. Getting 139's factors is done like this. The first two factors - and sometimes the only two - are the number itself, and 1.
Try 1. 1 x 139 = 139, so put these into our factor list.
1
...
139
Take 2... no good. 139/2 isn't a whole number. So we skip 2.
Now take 3... no. 139/3 isn't a whole number. So we skip 3.
Next try 4... no luck. 139/4 isn't a whole number. So we skip 4.
Now try 5... nope! 139/5 isn't a whole number. So we skip 5.
Try 6... no good. 139/6 isn't a whole number. So we skip 6.
Take 7... no. 139/7 isn't a whole number. So we skip 7.
Now take 8... no luck. 139/8 isn't a whole number. So we skip 8.
Next try 9... nope! 139/9 isn't a whole number. So we skip 9.
Now try 10... no good. 139/10 isn't a whole number. So we skip 10.
Try 11... no. 139/11 isn't a whole number. So we skip 11.
Take 12... no luck. 139/12 isn't a whole number. So we skip 12.
Now, since we don't have any more numbers to try, we're done!