Hi again guys,

I'm trying to take the derivative of the velocity model of a space shuttle:

v(t) = 0.001895t^3−0.08391t^2+13.69t+2.71

I get the acceleration as:

a(t) = 5.685E-3 t^2 - .16782 t + 13.69

I want to find the critical numbers so I want to set this function equal to zero. I go for the quadratic but I get a negative under the square root. Is there something I'm missing in this problem? Any clarification is appreciated