It is
(3-x)^2
i made sure it correct this time
Total cost is $38.16
Multiply 36 by 0.06 (2.16)
Add 2.16 to 36 (38.16)
Just like any other variable, we complete the same process

Now we can test it, let's say k=4, d=6, and c=2, let's find the value of g

OR
You didn't put the choices