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.