A particle of mass $1 \mathrm{g}$ moving with a velocity ${\overrightarrow{u}}_1=\left(3\hat{\mathrm{i}}-2\hat{\mathrm{j}}\right) \mathrm{m}\mathrm{ }{\mathrm{s}}^{-1}$ experiences a perfectly inelastic collision with another particle of mass $2 \mathrm{g}$ moving with a velocity ${\overrightarrow{u}}_2=\left(4\hat{\mathrm{j}}-6\hat{\mathrm{k}}\right) \mathrm{m}\mathrm{ }{\mathrm{s}}^{-1}$ . The velocity (in $\mathrm{m}\mathrm{ }{\mathrm{s}}^{-1}$ ) of the combined particle is