If the radius of the first Bohr orbit is x then the radius of the third orbit would be

All you need to know in order to answer this question is that the radius of an orbit depends on the principal quantum number, n, that describes the energy level on which the electron is located.

If you take a0 to be the Bohr orbit, you can say that you have

rn=n2⋅a0

In your case, the third orbit is characterized by

n=3

so you will have

r3=32⋅a0=9⋅a0