Or if you're interested in actually writing the function yourself seeing to how it is an assignment (presumably for school?), drawing a filled circle can be donw by iterating over every pixel in a square where the side is the diameter of the circle. You then check each pixel's distance from the center of this square (d^2 = x^2 + y^2) and if it is less or equal to the radius of the circle you set that pixel to whatever fill colour you want for the circle, otherwise you leave it be.
Note however that doing this using the dbDot() function will be very slow for larger circles.
If you want to do it this way I'd suggest either making a memblock image or to manipulate the pixel data of an image directly.