Answer
4.4 years.
Work Step by Step
If c denotes the speed of light or a radio wave, then $d = 4.2 \times 10^{16} m = ct$. Solve for t = d/c.
$$\frac{4.2 \times 10^{16} m }{300,000,000 m/s} = 1.4 \times 10^{8} s$$
There are (3600 s/hour)(24 hr/day)(365 day/year) = 31,536,000 seconds in a 365-day year.
$$\frac{1.4 \times 10^{8}s}{31536000 s/year} = 4.4 years.$$
