As ordered pairs ( g , C ) where g is the number of games and C is the cost
 ( 5, 20.50)   and ( 9, 28.50)
 the slope M =  ( 28.50 - 20.50 ) / (9-5)
                  =  8/4
                  = 2
So the slope M=$2  per game
Using (5, 20.50)
The intercept B = y - m* g
                        = 20.50 - 2 * 5
                         = 20.50 - 10
                         = 10.50
So the fixed base cost, or FLAT RATE is $10.50.
That is if they played ZER0 games, they still have
 to pay $10.50 just to get in.
The linear function is C (g)  = 2*g + 10.50