You need to change the mixed number into a improper fraction:
[tex]3\dfrac{1}{3}=\dfrac{3\cdot3+1}{3}=\dfrac{10}{3}\\\\2\dfrac{1}{4}=\dfrac{2\cdot4+1}{4}=\dfrac{9}{4}\\\\3\dfrac{1}{3}\cdot2\dfrac{1}{4}=\dfrac{10}{3}\cdot\dfrac{9}{4}\\\\=\dfrac{5}{3}\cdot\dfrac{9}{2}=\dfrac{5}{1}\cdot\dfrac{3}{2}=\dfrac{15}{3}=7\dfrac{1}{2}=7.5[/tex]