Error while trying to image fourier transform of a function?

Hello the code is the following:

syms x 
x=-400:400;
Y(x)=1./(j*x+3);
abs(Y(x))

I am trying to image abs of function Y(x) where Y(x) is Fourier's transform of function h(t)=e^(-3t)*u(t) for x=[-400,400].

Everytime i get the following error message: Error using sym/subsasgn (line 920)

Invalid indexing or function definition.

Indexing must follow MATLAB indexing. Function

arguments must be symbolic variables, and

function body must be sym expression.

Please help!

Answer 1

[madhan ravi]

x=-400:400;
Y=1./(j*x+3);
abs(Y)