I had this practice question and i would really help on how to solve it;

For each planet in the solar system, its year is the time it takes the planet to revolve around the center star. The formula E(x) = 0.2x^3/2 models the number of Earth days in a planet's year, E, where x is the average distance of the planet from the center star, in millions of kilometers. There are approximately 88 Earth days in the year of the planet Mercury. What is the average distance of Mercury from the center star? Round to the nearest million kilometers. Clearly state your final answer in a complete sentence with proper grammar and correct spelling.