I presume I have corrected the equation correctly?

The distance is given by s, so

so you were right. BUT you are looking for the distance from the starting point, so your answer is 40. (The negative sign means that, if you are plotting this on a number line, the object's position is on the left of the origin.

We want a t for s = 0. Thus

So either

or

The t = 0 solution is obviously true, since this is the starting point. So we are looking for the t = 5.

-Dan