Answer
a. Converging
b. 281 cm
Work Step by Step
a. The combination is converging because the net focal length is positive. The converging lens has a smaller magnitude of focal length.
b. See Chapter 23, Problem 87. The result quoted there relates the net focal length to the individual focal lengths.
$$f_T=\frac{f_1f_2}{f_1+f_2}=\frac{(-27.8cm)( +25.3cm)}{ -27.8cm +25.3cm }=281 cm$$
