Ineed this now i really dont understand this

Okay so I am very new to circles however I decided to give this one a shot for you. And I feel VERY strongly that it is not C. I believe it is A.
I think this because with circles the equation is
(x-h)^2+(y-k)^2=r^2
The h variable is the x coordinate at the center of your circle. The k variable is the y coordinate.
In this question it is stating that the origin is the center which gives you (0,0). It also states the radius, r=5.
Therefore when you put in all your known values into the equation you get,
(x-0)^2+(y-0)^2=5^2
And just removing the unnecessary zeros and simplifying things you get,
x^2+y^2=5^2

The discount is 8.70 . you dont tell us the percent of tax added so there is no way to find out, unless i'm not seeing anything. without tax its 37.70