To complete the equation \( y = kx \), where \( k \) is the constant of proportionality, you need to know the constant that relates the cost of ice cream custard to its weight in ounces.
If, for example, the cost is $2 per ounce, the equation would be:
\[ y = 2x \]
Here, \( k \) (the constant of proportionality) is 2, meaning the cost \( y \) in dollars is equal to 2 times the weight \( x \) in ounces.
Please provide the value of the constant of proportionality if you have it, and I can complete the equation for you!