I strongly recommend that you find an illustration of an ellipse that features the three distances a, b and c. You could Google "ellipse" and sort through the various illustrations that result, until you find the "right one."
There is an equation that relates a, b and c for an ellipse. It is a^2 = b^2 + c^2.
a is relatively easy to find. It is the distance from the center (0,0) of your ellipse to the right-hand vertex (20,0). So a = 20.
b is the distance from the center (0,0) of your ellipse to the right-hand focus (16,0). So b = 16. You could stop here, as it was your job to find b.
Or you could continue and find a also. a^2 =b^2 + c^2, so
here a^2 = 16^2 + 20^2. Solve this for a.