When the denominators can't divide into each other, you multiply each fraction by the other fractions' denominator to get a common denominator
It looks like this:
[tex] \frac{2}{3}*( \frac{4}{4} )= \frac{8}{12} [/tex]
[tex] \frac{3}{4}*( \frac{3}{3})= \frac{9}{12} [/tex]
now each denominator is 12