the best thing to do is to find a common denomiantor, and you can do this by listing the multiples.
5- 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85
17- 17 34 51 68 85 102
The lowest common multiple is 85, so you've got to multiply the first fraction by 17, and the second by 5.
3x17= 54, 5x17= 85 and 3x5= 15, 5x17= 85.
this then becomes 54/85 and 15/85, when you're adding fraction, you add the denominators- 54+15= 69, therefore, the fraction becomes 69/85
Hope this helps :)