Refractive index μ = Real depth / Apparent depth. Let the real depths from the two sides be $d_1$ and $d_2$. Apparent depths are 6cm and 4cm. So, $1.5 = d_1/6 \Rightarrow d_1 = 9$ cm. And $1.5 = d_2/4 \Rightarrow d_2 = 6$ cm. The total thickness is $d_1 + d_2 = 9 + 6 = 15$ cm.