Non-Elementary Integration · Medium problem

Use Frullani's theorem, $\displaystyle\int_0^{\infty}\dfrac{f(ax)-f(bx)}{x}\,dx=\bigl(f(0)-f(\infty)\bigr)\ln\dfrac{b}{a}$, to evaluate $\displaystyle\int_0^{\infty}\dfrac{\tanh 3x-\tanh x}{x}\,dx$.