x=t^2

y=t^3-3t

a) Show that C has two tangents at the point (3,0) and find their equation.

b) Find the point on C where the tangent is horizontal or vertical.

c) Determine where the curve is concave upward or downward?

I was able to do part a and b, but I'm stuck on part c. How do you find the second derivative of this curve?