Physics help.. optics find the minimum length of the camera tube?

A photographer has a simple camera. The camera has a single convex lens of focal length 50.0 mm. The lens may be moved relative to the film plane such that the lens-film distance may be varied between 50.0 mm and 70.0 mm
1)Find the range of object distances for which the camera may be adjusted to give a sharp image on the film. For the nearest possible object distance find the linear magnification
2)The photographer finds that it is necessary to focus on an object at 120 mm in front of the lens. In order to produce a sharp image on the film the photographer reads in a book that he must insert an extension tube of length x between the lens and the camera such that the range of lens-film distances available becomes (50.0 + x) mm to (70.0 + x) mm Find the minimum length ofthis tube. Investigate the effect of this tube on the furthest object that may be imaged by the camera. What effect has adding the tube had on the linear magnification for the nearest object distance compare to (1)

Using the convex formula will help
1/f = 1/u + 1/v
1) maximum range will be at infinity as this will focus the image at 50mm
closest is when lens is extended out to 70mm giving v= 70 so with f = 50
1/50 = 1/u + 1/70
1/u = 1/50 - 1/70
u = 175mm approx.