One of the basic equations in physics is Speed = Distance/Time. Thus,
Distance = Speed*Time
Time = 1 light-year * 365 days/yr * 24 h/day * 3,600 s/h = 31536000 seconds
Distance = (3×10⁸ m/s)*(31536000 s) = 9.4608×10¹⁵ meters
Convert meters to miles:
Distance = 9.4608×10¹⁵ m * 1 mile/1,609 m = 5.88×10¹² miles