The two light beams having intensities I and 9I interfere to...

Resultant intensity is given by, ${I}_{resultant} = {I}_{1} + {I}_{2} + 2\sqrt{{I}_{1}{I}_{2}}cosϕ$

At $P$, ${I}_{P} = 9I + I + 2\sqrt{9I \times I}\cos \left(\dfrac{\pi}{2}\right) = 10I$

At $Q$, ${I}_{Q} = 9I + I + 2\sqrt{9I \times I}\cos \left(\pi\right) = 10I - 2 \times 3I = 4I$

Now, difference between intensities is $10I - 4I = 6I$