At the farmers market you can buy 3 melons for $10.50, 6 melons for $21, or 9 melons for $31.50. What is the constant of proportionality for buying melons?

If you buy 3 melons for $10.50, so one melon costs $10.50:3=$3.50.

If you buy 6 melons for $21, so one melon costs $21:6=$3.50.

If you buy 9 melons for $31.50, so one melon costs $31.50:9=$3.50.

The constant of proportionality is 3.50.