What is the equation of an ellipse with a center (-4,2) and a horizonal major axis?

I know the equation is x^2/a^2+y^2/b^2 and that's all.

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you will have to give more information.
With only the centre (-4,2) all we can say is
(x+4)^2/a^2 + (y-2)^2/b^2 = 1