Quick Solve - Literally Stuck at the Last Step

[IntegrateMe]

The position of a particle moving in the xy-plane is given by the parametric equations x = t³ - 3t² and y = 2t³ - 3t² - 12t. For what values of t is the particle at rest?

So we're trying to find when the slope (dy/dx) is equal to 0.

dy = 6t² - 6t - 12
dx = 3t² - 3t

dy/dx = (6t² - 6t - 12) / (3t² - 3t)
dy/dx = [6(t² - t - 2)] / [3t(t - 2)]
dy/dx = [2(t + 1)(t - 2)] / [t(t -2)]
dy/dx = 0 @ t = -1 or 2

So, i would assume the answer is "-1 or 2" but the answer is really just "2 only". Can anyone explain this to me?

 

[Gregg]

Assuming t is the time, if you find multiple values in the result you will discard the ones that are not useful, like the idea of something taking a negative amount of time.

 

[IntegrateMe]

Wow, that was simple. I'm sorry but i should've seen how obvious that was. Thanks a bunch!