Answer
(a) farther
(b) $2.36\times 10^7m$
Work Step by Step
(a) We know that $T\propto r^{\frac{3}{2}}$. This shows us that the orbital period and radius are directly proportional. Thus, Deimos is farther from Mars than Phobos because it has the greater orbital period.
(b) The required distance can be determined as:
$r=(\frac{T\sqrt {GM_M}}{2\pi})^{\frac{2}{3}}$
We plug in the known values to obtain:
$r=(\frac{(1.10\times 10^5)\sqrt{(6.67\times 10^{-11})(6.45\times 10^{23})}}{2\pi})^{\frac{2}{3}}$
$r=2.36\times 10^7m$
