First we can simplify the result
[tex]\frac{3}{15}=\frac{1}{5}[/tex]in order to know the missing fraction, we will call x
[tex]\begin{gathered} x=\frac{1}{5}-\frac{3}{10} \\ x=-\frac{1}{10} \end{gathered}[/tex]we can check it
[tex]-\frac{1}{10}+\frac{3}{10}=\frac{2}{10}=\frac{1}{5}=\frac{3}{15}[/tex]