When is the particle speed up/slowing down

[betsinda]

Homework Statement

The postion of a particle is given by s=2t^2-9t^2 where t is in seconds and s in meters

a) when is the particle speeding up/slowing down

Homework Equations

The Attempt at a Solution

I have calculated the
1st derivative (velocity) as 6t^2-18t
2nd derivative (acceleration) as 12t-18

and I know that the particle is speeding up when the velocity and the acceleration have the same sign and slowing down when they have opposite signs. However I am not sure how to express this to answer the question.

Thanks

 

[yyat]

Find the values of t for which the velocity and for which the acceleration is positive/negative. Hint: They can only change sign at the zeros.