To determine a denominator that would make the ratios \( \frac{213}{49} \) proportional to \( \frac{a}{b} \), we need to find a value \( b \) such that:
\[ \frac{213}{49} = \frac{a}{b} \]
However, you did not specify a value for \( a \). Proportionality means that the two ratios are equal, so we can express \( b \) in terms corresponding to given \( a \):
\[ b = \frac{49 \cdot a}{213} \]
If you provide a specific value for \( a \), I can calculate the corresponding value of \( b \). Please provide the value of \( a \) or clarify your question!