You have to find the denominator which matches but is the smallest it’s kinda hard to explain I can give an example though
So 3/5 4/6 you need to find the smallest possible denominator that matches so
5x1=5 5x2=10 5x3=15 5x4=20 5x5=25 5x6=30
And you do that for 6’s too
6x1=6 6x2=12 6x3=18 6x4=24 6x5=30
You find the number that you see matches first which is 30