Once you have found the common denominator, you must find the numerator for the equivalent fractions.
To do this, then for each fraction in turn, first divide the common denominator by the original denominator, then multiply this by the original numerator.
i.e. for 2/7 + 3/5, using a common denominator of 35.
Therefore 2/7 + 3/5, can be written using equivalent fractions as, 10/35 + 21/35.
10/35 + 21/35 = 31/35 .
(this fraction is already in its lowest form).