Answer:
Approximately [tex]31[/tex] years, assuming that each year includes approximately [tex]365.25[/tex] days.
Explanation:
Make use of the following facts about the units:
Divide total distance for a round-trip [tex]2 \times (5.9 \times 10^{9}\; {\rm km})[/tex] by speed to find the time required. As requested, leverage dimensional analysis to ensure that the unit of the result is in year. Make sure the intermediary units in the numerators match those in the denominators.
[tex]\begin{aligned}t =\; &\frac{s}{v} \\ =\; & \frac{2 \times (5.9 \times 10^{9}\; {\rm km})}{12 \; {\rm km\cdot s^{-1}}} \\ \approx\; &\frac{2 \times (5.9 \times 10^{9})}{12}\; {\text{s}} \\ &\times \frac{1\; \text{hour}}{3600\; \text{s}} \times \frac{1\; \text{day}}{24\; \text{hour}} \times \frac{1\; \text{year}}{365.25\; \text{day}} \\ =\; & \frac{2 \times (5.9 \times 10^{9})}{12 \times 3600 \times 24 \times 365.25}\; {\text{year}}\\ \approx\; & 31\; \text{year}\end{aligned}[/tex].