Resultant wave equation

$=A \sin \omega t+A \sin \left(\omega t-\frac{\pi}{4}\right)+A \sin \left(\omega t+\frac{\pi}{4}\right)$

$=\mathrm{A} \sin \omega \mathrm{t}+\sqrt{2} \mathrm{A} \sin \omega \mathrm{t}$

$=(\sqrt{2}+1) \mathrm{A} \sin \omega \mathrm{t}$

Resultant wave amplitude $=(\sqrt{2}+1) \mathrm{A}$

as $I \propto A ^{2}$

so $\frac{\mathrm{I}}{\mathrm{I}_{0}}=(\sqrt{2}+1)^{2}$

$I=5.8 \mathrm{I}_{0}$