Solución
Recordemos que la ecuación de una elipse de centro <img src="https://tex.z-dn.net/?f=h%2Ck" /> y eje focal paralelo al eje y es <img src="https://tex.z-dn.net/?f=%5Cfrac%7B%28x-h%29%5E%7B2%7D%7D%7Bb%5E%7B2%7D%7D%2B%5Cfrac%7B%28y-k%29%5E%7B2%7D%7D%7Ba%5E%7B2%7D%7D%3D1" />
Además pasa por el punto <img src="https://tex.z-dn.net/?f=A%3D%280%2C2%29" /> y centro <img src="https://tex.z-dn.net/?f=C%3D%282%2C4%29" />
Entonces sustituyendo los datos en la ecuacion dada se obtiene :
<img src="https://tex.z-dn.net/?f=%5Cfrac%7B%280-2%29%5E%7B2%7D%7D%7Bb%5E%7B2%7D%7D%2B%5Cfrac%7B%282-4%29%5E%7B2%7D%7D%7Ba%5E%7B2%7D%7D%3D1%5C%5C%5Cfrac%7B%28-2%29%5E%7B2%7D%7D%7Bb%5E%7B2%7D%7D%2B%5Cfrac%7B%28-2%29%5E%7B2%7D%7D%7Ba%5E%7B2%7D%7D%3D1%5C%5C%5Cfrac%7B4%7D%7Bb%5E%7B2%7D%7D%2B%5Cfrac%7B4%7D%7Ba%5E%7B2%7D%7D%3D1%5C%5C4a%5E%7B2%7D%2B4b%5E%7B2%7D%3Da%5E%7B2%7Db%5E%7B2%7D" />
Despejando <img src="https://tex.z-dn.net/?f=a" /> se obtiene :
<img src="https://tex.z-dn.net/?f=4a%5E%7B2%7D%3Da%5E%7B2%7Db%5E%7B2%7D-4b%5E%7B2%7D%5C%5Ca%5E%7B2%7D%3D%5Cfrac%7Ba%5E%7B2%7Db%5E%7B2%7D-4b%5E%7B2%7D%7D%7B4%7D%3D%5Cfrac%7Bb%5E%7B2%7D%28a%5E%7B2%7D-4%29%7D%7B4%7D%5C%5Ca%3D%5Csqrt%7B%5Cfrac%7Bb%5E%7B2%7D%28a%5E%7B2%7D-4%29%7D%7B4%7D%7D%5C%5Ca%3D%5Cfrac%7Bb%7D%7B2%7D%5Csqrt%7Ba%5E%7B2%7D-4%7D" />
De manera similar despejando <img src="https://tex.z-dn.net/?f=b" /> nos queda :
<img src="https://tex.z-dn.net/?f=b%3D%5Cfrac%7Ba%7D%7B2%7D%5Csqrt%7Bb%5E%7B2%7D-4%7D" />
Finalmente la ecuación buscada es :
<img src="https://tex.z-dn.net/?f=%5Cfrac%7B%28x-2%29%5E%7B2%7D%7D%7Bb%5E%7B2%7D%7D%2B%5Cfrac%7B%28y-4%29%5E%7B2%7D%7D%7Ba%5E%7B2%7D%7D%3D1%5C%5C%5Cfrac%7B%28x-2%29%5E%7B2%7D%7D%7B%5Cfrac%7Ba%5E%7B2%7D%7D%7B4%7D%28b%5E%7B2%7D-4%29%7D%2B%5Cfrac%7B%28y-4%29%5E%7B2%7D%7D%7B%5Cfrac%7Bb%5E%7B2%7D%7D%7B4%7D%28a%5E%7B2%7D-4%29%7D%3D1" />
Saludos.