Could someone please help me with this problem?

An object moves back anf forth along a straight line, with distance, s, in inches from its starting point, given by the equation.

$\displaystyle s=10^{2}-50t $

a) How far is it from its starting point after 4 seconds?

Why do I get -40?

b) How long will it take before it returns to its starting point?

Thank you!