I need help, it seems obvious but I am having difficulty with finding the answer.?

You use Kepler's 3rd Law, a^3=p^2, where a^3 is the distance from the sun cubed in AU, and p^2 is the orbital period in years squared. So 4^3=p^2=64. So p=sqrt of 64, which is 8. So it takes the asteroid 8 years to orbit the sun.