How to use fplot function???

Below is what I've written in Matlab workspace,

syms x; f(x)=input('f(x)='); fplot(f,[4,9]);

But there are only errors : ??? The following error occurred converting from sym to double: Error using ==> mupadmex Error in MuPAD command: DOUBLE cannot convert the input expression into a double array.

If the input expression contains a symbolic variable, use the VPA function instead.

So what is the VPA function and Mupad error anyway??

(I'm a begginer, please help me)