Answer:
Constant of proportionality: 
Equation: 
Step-by-step explanation:
By definition, Direct proportion equations have the following form:

Where "k" is the Constant of proportionality.
In this case, let be "c" the the amount of caffeine consumed (in mg) from a glass of Diet Pepsi and "d" the number of ounces that was drank.
So, the equation that represents this relationship will have this form:

Then, the first step is to find the Constant of proportionality "k".
Knowing that:

We can substitute values into the equation:

Now, solving for "k", we get:

Therefore, we can write the following equation that represents that proportional relationship:
