There's countless of fractions that can be equivalent to 6/5. Multiplying the numerator and denominator with the same number will always give an equivalent fraction. For example, let's say I wanted to multiply 3 to both the numerator and denominator.
(6 * 3)/(5 * 3) = 18/15
To check if it's equivalent, set it equal to 6/5, like so:
18/15 = 6/5
Then when you set 6/5's denominator equal to 15, you will see that the numerator becomes 18, and that it is an equivalent fraction.
You can also divide both the numerator and the denominator by the same number. 6/5 doesn't have any common factors besides 1, so I'm going to use another fraction as an example. Let's say we are given 8/12. Both of these numbers have a common factor of 4. Let's divide these both by that number.
(8/4)/(12/4) = 2/3
When setting 2/3 equal to 8/12 and getting common denominators, you will also see that 2/3 is equal to 8/12, and therefore, an equivalent fraction.
HOWEVER, you cannot add or subtract by the same number in both the numerator and the denominator, because they will not be equivalent fractions to the original fraction that was given. Adding 3 to both sides of 6/5 will give 9/8, which is slightly smaller than 6/5. And subtracting both sides will give 3/2, which is slightly larger than 6/5.