I have a question about a finite σ measure on $({\mathbb{R}}^{\mathbb{+}},B({\mathbb{R}}^{\mathbb{+}}))$:

$Show\text{}that\text{}2{\int}_{{\mathbb{R}}^{\mathbb{+}}}x\mu ([x,\mathrm{\infty}[)dx={\int}_{{\mathbb{R}}^{\mathbb{+}}}{x}^{2}\mu (dx)$

I know that I should use fubini, but unfortunately I don't know where to start. Any help welcome.

$Show\text{}that\text{}2{\int}_{{\mathbb{R}}^{\mathbb{+}}}x\mu ([x,\mathrm{\infty}[)dx={\int}_{{\mathbb{R}}^{\mathbb{+}}}{x}^{2}\mu (dx)$

I know that I should use fubini, but unfortunately I don't know where to start. Any help welcome.