Please help me with thisTHANKS TO ALL HELPFUL USERS

If John charge $12                   then he has 14 students,         and he gets $ 14*12,
If he increases on $2(12+2)  , he loses 1 student (14-1),           gets (12+2)*(14-1)
if he increases on                    he loses 2 students (14-2)         gets (12+2*2)(14-2)
2+2 (12+2*2) 
if he increases 2*3          he loses 3 students, (14-3)         gets (12+2*3)(14-3)
2x*3  (12+2*3)
and so on......
if he increases 2*x,        he loses x students, (14-x)left,   gets (12+2x)(14-x)
(12+2x)

So , his revenue in general =
(12+2x)(14-x)=12*14-12x+28x-2x²=-2x²+16x+168

I think it should look like this
c(x)= -2x²+16x+168