How can i insert a value for x in f(x)=(x^3/3)-(2*x^2)+4*x

Just replace x, wherever it occurs in f(x), by whatever value you are interested in and evaluate.

 

For example if x = 2, then f(2) = 2^3/3 - 2*2^2 + 4*2 = 8/3 - 2*4 +8 = 8/3