Okay, here are the steps to solve this problem:
1) Use the mirror equation: 1/d0 + 1/di = 1/f
2) Given: d0 = 0.10 m, di = 3.00 m
3) Set up and solve the mirror equation for f:
1/0.10 + 1/3.00 = 1/f
1/f = 0.1 + 0.3333... = 0.4333...
f = 1/0.4333... = 2.30 m
4) For a concave mirror, R = 2f
R = 2 * 2.30 = 4.60 m
5) Use the magnification equation: m = -