Fixed Issue 40893 Graphing Circles

The equation of a circle is (X-H)^2 + (Y+K)^2=R^2. Because there is a "+" in one term and a "-" in the other term I had to switch the pluses and minuses within the java script to give the user the correct center according to the above equation. I have tried all the possible combinations of pluses, minuses, and zeroes and the exercise works.