LCD is least common divisor
7.5/7 - 2/3
Here the denominators are 7 and 3 the lowest common divisor of these is 7 x 3 = 21
Make both denominators 21
7.5/7 x 3/3 - 2/3 x 7/7
= 22.5/21 - 14/21
= 8.5/21
Somehow, I think the 7 is the problem number. Am I right?
In any case, has denominators of 7 and 3.
This one is easy since they are both prime numbers. You just multiply them together to get 21 as your new denominator. Now convert each fraction in your subtraction expression to the new denominator.
and
Determine what number you need to multiply the denominator by to get the new denominator. In the first one, it would be 3.
In the second one, it would be 7.
Now use your two converted fractions to obtain the difference.