So I need reassurance for the first answer and the other two I don't know how to do from my homework sheet, please

So I need reassurance for the first answer and the other two I don't know how to do from my homework sheet, please

help.

a. use the derivative of f(x)= 3x^3+ 9x to determine any points on the graph of f(x) at which the tangent line is horizontal.

so I got (1,12) and (-1,-12) I dont know if thats right

b. If f(x)=-3x^2-4x, find the following limit, if it exist.

lim h-->0 f(x+h)-f (x)/ h

c. Use the function below and its derivative to determine any points on the graph of f at which the tangent line is horizontal.

f(x)=3x^4 -6x^2, f ' (x)= 12x^3 -12x