The object distance is given, it is 40 cms away from the convex lens.
Now the light from the source will meet the convex mirror after getting refracted from the convex lens, and then light will retrace its path. To retrace its path, the light must fall on the convex mirror in such a way that the rays would meet it's centre of curvature if extended.
So radius of curvature =2f=2*7.5=15 cm
So object distance =-40 cm
Image distance = d+15 cm