\(\frac{1}{0.12}+\frac{1}{2.4}=\frac{1}{f} \Rightarrow \frac{1}{f}=\frac{210}{24}\)

Upon putting the glass slab, shift of image is

\(\Delta x = t \left(1-\frac{1}{\mu}\right)=\frac{1}{3}\,cm\)

Now \(v =12-\frac{1}{3}=\frac{35}{3}\,cm\)

Again apply lens formula

\(\frac{1}{0.12}+\frac{1}{ u }=\frac{1}{ f }=\frac{210}{24}\)

Solving \(u =-5.6\,m\)

Thus shift of object is

\(5.6-2.4=3.2\,m\)