Do you just want to plot it? Why would you want to differentiate?
The lowest point will be x = 0 at t = 0, and it will keep rising after that.
I recommend only plotting it from t = 0 to t = 6. It rises too fast after that to be worth plotting.
Include these points, and draw a smooth line through them:
(0,0)
(1,5.75)
(2,16)
(3,35.25)
(4,38)
(5,118.75)
(6,192)
