A square aluminium (shear modulus is $25 \times 10^9 \mathrm{Nm}^{-2}$ ) slab of side 60 cm and thickness 15 cm is subjected to a shearing force (on its narrow face) of $18.0 \times 10^4 \mathrm{~N}$. The lower edge is riveted to the floor. The displacement of the upper edge is $\_\_\_\_$ $\mu \mathrm{m}$.