According to the question;
Focal length (f) = 15cm;
Radius of curvature(R=2f) = 30cm;
Image distance (u) = 30cm;
Since the image distance is 30 cm and the Radius of curvature of the lens is also 30 cm.
∴ The object must be placed at the radius of curvature (30cm) in front of lens, because when the object is kept at radius of curvature its image is also formed at radius of curvature on other side of the lens.
Figure below shows the image formation.
Object should be placed at 30cm in front of lens.