This graph shows a proportional relationship between the number of pizzas (p) a person buys and the total cost of pizza (c).

Find the constant of proportionality (r). Using the value for r, enter an equation in the form of c = rp that represents the relationship between the number of pizzas (p) and the total cost (c). Write the value of r in the form of a whole number or fraction.
(ill give brainliest to whoever has the best explanation or answer)