([x1-cx]/a)^2 + ([y1-cy]/b)^2 = 1

([x2-cx]/a)^2 + ([y2-cy]/b)^2 = 1

([x3-cx]/a)^2 + ([y3-cy]/b)^2 = 1

where x1, y1, x2, y2, x3, y3 and b are constants. Re-arranging we get:

cy = y1 +- SQRT(b^2[1 - [(x1-cx)/a]^2])

cy = y2 +- SQRT(b^2[1 - [(x2-cx)/a]^2])

cy = y3 +- SQRT(b^2[1 - [(x3-cx)/a]^2])

y1 + SQRT(b^2[1 - [(x1-cx)/a]^2]) = y2 + SQRT(b^2[1 - [(x2-cx)/a]^2]) = y3 + SQRT(b^2[1 - [(x3-cx)/a]^2])

how can i solve to get the values of cx,cy and a,