What Coldplay said is correct, but just in case you still do not understand. I will show several examples.
If the dominators of the two fractions are different, you have to make the dominators the same and then subtract the two fractions.
\(\frac{2}{3}-\frac{3}{5}\)
\(\frac{10}{15}-\frac{9}{15}\)
\(\frac{1}{15}\)
If the dominators of the two fractions are the same, leave the dominators alone and subtract the two fractions.
\(\frac{2}{3}-\frac{1}{3}\)
\(\frac{1}{3}\)
This will even work with adding fractions togother.
If the dominators of the two fractions are different, you have to make the dominators the same and then add the two fractions.
\(\frac{2}{3}+\frac{3}{5}\)
\(\frac{10}{15}+\frac{9}{15}\)
\(\frac{19}{15}\) or \(1\frac{4}{15}\)
If the dominators of the two fractions are the same, leave the dominators alone and add the two fractions.
\(\frac{2}{3}+\frac{1}{3}\)
\(\frac{3}{3}\) or \(1\)