If the center is at (0, 0) and the vertex is at (20, 0), then the distance, a, is the length from the center to the vertex, 20. The distance from the center to the focus is c. The distance from the center to the focus is 16, so c = 16. The formula we use to find the focus is 
. We have our c value and our a value, so we will sub in those to find b. 
and 256 = 400 - b^2. -b^2 = -144, so b = 12. There you go!