Just need a check
the amount a car dealership earns for selling a car is directly proportional to the price of the car. the table shows this relationship.
what is the constant of proportionality?
car price ($) (x) 20,000 30,000 40,000
amount earned ($) (y) 1000 1500 2000
my answer : 14.5
right or wrong

Wrong. 
If it's a matter of directly proportional, then all you need is one single point.

Look at the first one:

x = 20,000
y = 1,000

y/x = 1,000/20,000 = 0.05

y = 0.05 x

The constant of proportionality is 0.05  (5 percent).

Look at the other points just to check it out and see if it holds water:

x = 1,500
y = 30,000
x/y  =  0.05  yes !

x = 2,000
y = 40,000
x/y  =  0.05  yes !

It checks out. 
The constant of proportionality is 5% for all points in this table.

Idon’t know how to do that