According to your answer, it looks like you could use some help with geometry. We recommend starting with Volume 4 of the Learn Math Fast System. That book will teach you how to find the circumference and the area of a circle. Plus a lot more geometry.
RESULTS: The end of 7th Grade Level, however, the Learn Math Fast System is for all ages. Volume 4 covers 3rd - 8th grade geometry topics, but there is no mention of grade level within the books.