Two vectors a = a_1 i + a_2 j + a_3 k and b = b_1 i + b_2 j + b_3 k are collinear if

Two vectors $\displaystyle \overrightarrow{\mathrm{a}}=\mathrm{a}_{1} \hat{\mathrm{i}}+\mathrm{a}_{2} \hat{\mathrm{j}}+\mathrm{a}_{3} \hat{\mathrm{k}}$ and $\displaystyle \overrightarrow{\mathrm{b}}=\mathrm{b}_{1} \hat{\mathrm{i}}+\mathrm{b}_{2} \hat{\mathrm{j}}+\mathrm{b}_{3} \hat{\mathrm{k}}$ are collinear if