The ratio of trombones to violas is 1 to 3, that is, for every trombone there are 3 violas.
So, if there are 9 violas, the number of trombones is three, because 9 violas represents a triple proportion, which means we need to multiply by 3 to find the number of trombones according to the given proportion.
The expression that gives the number of trombones is
The number of trombones is 3, there are 3 trombones while there are 9 violas.