Right, where I would start off is to plot a graph of (-7,0) and (7,0). It says that q>0, so the centre is (p,q) where both are positive values. You can use pythagoras to determine the value of q by doing 25^2 - 7^2 = q^2, and take the square root of q. The value of p would have to be 0 because it's a circle, so the radius is always the same and that means the centre to (-7,0) and (7,0) must be the same.