Question

3/5 and4/7 put these over a common denominator

Answer 1

The common denominater for the question is 35. Multiply 7 x 5 because in this case, they have no number before that which is common.

Answer 2

A possible common denominator are ->   21/35 and 20/35.    

I just multiplied the denominators by each other, if I can't find another way. 
I would multiply five times sevon, to get 35, and then do the same to the numorator, getting 21. Then I would multiply sevon by five, getting 35, and then do the same to the numorator, getting 20. 

I hope this helps! XD