What is the essential bandwidth with the given function? Let the function be 

$$
f(t) = e^{-3t} u(t -1)
$$
The criterion is set at 70% of the total signal energy.

I know how to find total energy, but not how to find the the $W$ (or frequency) at which the signal is a certain amount.
Find the Fourier Transform $F(\omega)$ and perform the following integration.$\frac{1}{2\pi}\int_{-W}^{W}|F(\omega)|^2\mathrm{d}\omega=0.7E$, where $E$ is the total Energy of the function given by: $E=\frac{1}{2\pi}\int_{-\infty}^{+\infty}|F(\omega)|^{2}\mathrm{d}\omega$.The value of $W$ is the 70% Bandwidth.