Answer
$58$ million kilometers
Work Step by Step
Plug in the number of Earth days to the formula. Then divide by $0.2$ on both sides, and apply the exponent $\frac{2}{3}$ to both sides to simplify $x$.
$E=0.2x^\frac{3}{2}$
$88=0.2x^\frac{3}{2}$
$\frac{88}{0.2}=x^\frac{3}{2}$
$440=x^\frac{3}{2}$
$x=440^\frac{2}{3}$
$x\approx58$