Answer
a. -2.8x
b. The 5.5D lens
Work Step by Step
a, b. Use equation 25-3 to find the magnification of the telescope. The powers are the reciprocals of the focal lengths.
$$M=-\frac{f_o}{f_e}=-\frac{P_e}{P_o}$$
To obtain a magnification greater than 1, the higher power lens (5.5D) should be the eyepiece.
$$M=-\frac{5.5D}{2.0D}=-2.75\approx-2.8$$