If the radius of mars is about 13.7% of neptune's radius, what is the radius of neptune?

You said that M is 13.7% of N

In algebraic notation M = 0.137 N

Divide each side by 0.137 : N = M / 0.137 = 7.3 M (rounded)

Online or in a real book, you can look up the radius of Mars.

Then write it in place of 'M' in this solution, and you'll have

an actual number for the radius of Neptune.