Answer
The intensity $I$ is zero when $L = 2250~nm$
Work Step by Step
We can see that the intensity decreases from a maximum value when $L = 0$ to a minimum value of zero when $L= 750~nm$
The intensity will fluctuate between maximum value and zero as $L$ increases by $750~nm$
The intensity $I$ is a maximum when $L = 1500~nm$
The intensity $I$ is zero when $L = 2250~nm$
