Two ratios are equivalent when we can take one of the ratios and multiply or divide by a number in order to get the other.
In this case, we can multiply 3:6 by 6 and get:
[tex]\begin{gathered} 3\colon6 \\ 3\cdot6\colon6\cdot6 \\ 18\colon36 \end{gathered}[/tex]Since 18:36 is different from 18:8, we can say that those ratios are not equivalent.