Finding equation of Ellipse

Dec 2009
I have the following problem:

Find the equation of an ellipse with foci at \(\displaystyle (2\sqrt{3}+1,-7)\) and \(\displaystyle (1-2\sqrt{3},-7)\), and a semi-major axis 4 in the form (after shift)

\(\displaystyle \frac{x^2}{a^2}+\frac{y^2}{b^2}=1\)

Give the coordinates of the centre, xc and yc, the value of a and the value of b.

I don't really know how to go about solving this one.