Answer:
a. $18.36 for 3 pizzas  :  k  = 6.12
b. $4.17 for 3 pounds of bananas .   k  = 1.39
c. $16.48 for 4 pounds of potatoes .  k  = 4.12
d. 2 cups of flour to make 24 cookie .  k  = 12
Step-by-step explanation:
PROPORTIONALITY:
Two quantities x and y are said to proportional to each other
if for x ∝ y , x = y k.
Here, k is called the PROPORTIONALITY CONSTANT.
⇒ 
  
Now, for the given quantities:
a.  $18.36 for 3 pizzas [box]    
Here, 
So, the proportionality constant is 6.12.
b. $4.17 for 3 pounds of bananas [box]
Here, 
So, the proportionality constant is 1.39.
c. $16.48 for 4 pounds of potatoes [box]
Here, 
So, the proportionality constant is 4.12.
d. 2 cups of flour to make 24 cookies
Here,  
So, the proportionality constant is 12.