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.