Let:
x = Cost of each kilogram of honey drew
y = Cost of each kilogram of watermelon
Once, she used 3 kilograms of honeydew melon and 3 kilograms of watermelon, which cost her $12. So:

She also used 2 kilograms of honeydew melon and 1 kilogram of watermelon, spending a total of $6 on the melons. So:

From (2) solve for y:

Replace (3) into (1):

Replace x into (3):

Answer:
Honeydew melon costs 2 dollars per kilogram
Watermelon costs 2 dollars per kilogram