x. y 3 postcards for $5.40 6 postcards for $10.80 9 postcards for $16.20 To find the constant of proportionality, take y/x, so $5.4/3 $10.8/6 $16.2/9 They all equal $1.80/1. As a result, the constant of proportionality is $1.80 for 1 postcard. Hope it help!