Ineed i don't understand how to do this

On this you are really just trying to find the lowest number that both of the numbers go into. For example if you have the numbers 6 and 9, 18 would be the lowest common multiple because it is the smallest number that both the numbers go into. An example with fractions is 3/4 and 5/6, what they are trying to tell you is that you find a common denominator for the two fractions (here it would be 12, making them 9/12 and 10/12) and then you can add them together to get the least common multiple (for this problem it would be 19/12). This is a faster way that works for fractions, but would not work as well in my opinion for whole numbers.

Where are the calculations?