The question asks to solve for the length of the time interval the spaceship would have to complete one rotation on its axis, so isn't that finding the period, not the acceleration?
Homework Statement
A spaceship in outerspace has a donut shaped with a 500 m outer radius. The inhabitants stand with their heads towards the center and their feet on an outside rim. Over what time interval with the spaceship have to complete one rotation on its axis to make a bathroom scale...