f(x) in that context is the xth index of f. Indices must be positive integers (or logicals - don't worry about it if you don't know what that means), which x is not. In this case you don't need to index f. Just say:
x = 0:0.001:0.1*pi;
f = 5*acos(2*x) % etc etc the rest of your expression for f
Then you can plot f(x) vs x like:
plot(x,f)
