Hi, can someone help me with these?

A particle moves along the x-axis, its position at time t is given by x(t) = 9t / 3+8t^2, t>0 where t is seconds and x is meters

The acceleration of the particle equals 0 at time t=_____ and t=_____ seconds

So I found the second derivative to be this big mess:

(-1296t^2 - 6912t^4 - 9216t^6 - 2592t + 18432t^5) / (9 +48t^2 +64t^4)^2

^ I hope this is right, but maybe it's where I went wrong

then I know to set the equation equal to 0, but then I don't know what to do next, any help?

also I need help with this question:

The function y = e^rx satisfies the equation y'' - 7.4y' + 12y = 0 if r=____ or if r=____

no idea what to do here.

thanks in advance!