Position of Central Maxima due to multiple path differences in YDSE

In a YDSE experiment, the apparatus is immersed in a liquid of refractive index μ1. And also one of the slits (S2) is covered with a transparent slab of refractive index μ2 of thickness t as shown in the figure. I have been asked to find the position of central maxima on the screen. ($mu_{3} = 1$)

Attempt at a Solution:

The optical path difference at the central maxima must be zero.

$delta x = mu_{1} frac{y cdot d}{D} + t(1-mu_{2}) = 0
y = frac{Dt(mu_{2} -1)}{d mu_{1}}$

(y = distance of maxima from O,
D distance between the slit and the screen)

But my book gives the answer as:

$y = frac{Dt(mu_{2} -1)}{d}$

I can’t find the reason for the missing $mu{1}$. Please help me find the problem in my approach. Thanks in advance.