2f, 4f

It is given in the problem that both the mirrors have

equal focal length f.

They will form one image under two conditions.

Case 1: When the source is at distance '2f' from each mirror, that is the object is at center of curvature of the mirrors. Then the image will be produced at the same point S as shown in the figure.

Therefore,

d = 2f + 2f

Case 2: When the source s is at a distance f from each mirror, the rays from the source after reflecting from one mirror will become parallel and so these parallel rays after the reflection from the other mirror the object itself. So. only one image is formed

Here f + f = 2f.