A spherical mirror and a thin spherical lens have each a focal length of –15 cm. The mirror and the lens are likely to be

For a concave lens, the first focus is on the same side as the object and is negative.

In the case of a concave mirror, the focus is in front of the mirror and is negative. 

Since the spherical mirror and the thin spherical lens have the same focal length of –15 cm, they are both likely to be concave.