There are many many different answers for the equation of
an ellipse passing through (8,3) with a major axis of 20.
Scroll down and you'll see that each of the four ellipses 
graphed below passes through (8,3) and has a major axis of 
20.  You MUST have something else given besides that or 
else there are an infinite number of answers.  

If you want to tell me what else that was given which you did 
not not tell us, we will be glad to help you. But there is no 
one answer to the problem which you posted.

You may tell me the additional information which you left out
in the thank-you note form below, and I'll get back to you
buy email.

  <--the red ellipse graphed below
 
  <--the blue ellipse graphed below
 
  <--the green ellipse graphed below
 
  <--the black ellipse graphed below



Edwin