In many cases, you can find the lowest common denominator by simply multiplying the two numbers together. This method works in your example.
7 x 5 = 35
The two fractions would become 15/35 and 28/35
There will be times when this will not get you straight to the answer, eg find the lowest common denominator of 8 and 12. For this you need to list some multiples of the numbers until you get a number which appears in both lists:
8: 8, 16, 24
12: 12, 24
So, the lowest common denominator of 8 and 12 is 24.
