Please help me! i just need to know how to do it, you dont have to give me the answer!?

A concave spherical mirror can be used to project an image onto a sheet of paper, allow- ing the magnified image of an illuminated real object to be accurately traced. a) If you have a concave mirror with a focal length of 6.1 cm, where would you place a sheet of paper so that the image projected onto it is twice as far from the mirror as the object is? Answer in units of cm.