**I'm assuming you meant the LCM (Lowest common multiple), for the denominators**
First, to find the LCM, take the bigger number and see if the other denominator can fit into it.
5 x 2 = 10
10 x 1 = 10
As you can see, it does!
So to solve this problem, you'd have to convert the 2/5 to 4/10 to be able to either add or subtract 7/10.
2/5=4/10
Hope this helps!