Hello Yong Ting Shen,
The White noise spectrum is plotted as given below:
You have given Power Spectral Density function = 2.0 and the range of excitation frequency is 0.1 – 500Hz.
Thus Sx(f) = 2.0 and range of f is 0.1 - 500 Hz and you can use the plot function to plot the white noise spectrum graph as given below
t = 0.1:1:500;