First, you break down the numbers. Find what numbers can be multiplied together to create those numbers. Now, you can see what factors the numbers have in common. For example, you have the two numbers 6 and 9. 6×1=6 2×3=6 and then 1×9=9 and 3×3=9. Both of the numbers have the factors 3 so they have the common factor of 3.