What is the missing denominator in the addition sentence 15/28 + 9/

To find the missing denominator in the addition sentence \( \frac{15}{28} + \frac{9}{x} \), we typically need to have a common denominator in order to add the fractions.

The denominator for the first fraction is 28. To express \( \frac{9}{x} \) with the same denominator, we can set \( x = 28 \) (the simplest choice), which allows us to rewrite the expression as follows:

\[
\frac{15}{28} + \frac{9}{28} = \frac{15 + 9}{28} = \frac{24}{28}
\]

Thus, if we are simply looking for an \( x \) that will allow the fractions to be added together, then the missing denominator \( x \) can be:

\[
x = 28
\]

So, the missing denominator in the addition sentence \( \frac{15}{28} + \frac{9}{\_} \) is \( 28 \).