Answer
$d=50nm$
Work Step by Step
As the reflection causes a phase difference of $\pi$ hence the path difference is $\pi$ or $\frac{\lambda}{2}$. That is:
$\Delta L=\frac{\lambda}{2}$
$7d-2d=\frac{\lambda}{2}$
$5d=\frac{\lambda}{2}$
$10d=\lambda=500nm$
$d=\frac{500nm}{10}=50nm$
