If there's one number by which you can multiply both the "top" and "bottom" of a given fraction to successfully turn it into another given fraction, then those two fractions must be equivalent.
To turn 2/7 into 6/21 we simply multiply BOTH the top AND bottom of that first fraction by 3. So the pair of fractions are equivalent fractions.
To turn 3/5 into 6/25 we would need to multiply the numerator by 2, but the denominator by 5. Because this would need different numbers for multiplying the top and bottom, we have to conclude that 3/5 and 6/25 are NOT equivalent fractions.
