To calculate the image position when given magnification by a concave mirror, you can use the mirror equation: 1/f = 1/d_o + 1/d_i, where f is the focal length of the mirror, d_o is the object distance, and d_i is the image distance. Magnification, M, is also given by -d_i/d_o. By substituting the values of magnification and focal length into the mirror equation, you can solve for the image distance and then determine the image position.