Answer
The phase difference is $~~0.80$
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 phase difference is $1.0~\lambda$ when $L = 1500~nm$
We can find the phase difference $x$ when $L = 1200~nm$:
$\frac{1200~nm}{1500~nm} = \frac{x}{1.0~\lambda}$
$x = 0.80~\lambda$
The phase difference is $~~0.80$
