Write the value of

Write the value of $\lambda$ so that the vectors $\vec{a}=(2 \hat{i}+\lambda \hat{j}+\hat{k})$ and $\overrightarrow{\mathrm{b}}=(\hat{\mathrm{i}}-2 \hat{\mathrm{j}}+3 \hat{\mathrm{k}})$ are perpendicular to each other.