Multiply :

2a³ - 3a²b and $-\dfrac{1}{2}$ ab²

$(2a^3 - 3a^2b) \times \Big(-\dfrac{1}{2}ab^2\Big)\\[1em] = \Big[2a^3 \times (- \dfrac{1}{2}ab^2) - 3a^2b \times (- \dfrac{1}{2}ab^2)\Big]\\[1em] = - \dfrac{1 \times 2}{2}a^{3+1}b^2 + \dfrac{1 \times 3}{2}a^{2+1}b^{1+2}\\[1em] = - \dfrac{2}{2}a^4b^2 + \dfrac{3}{2}a^3b^3\\[1em] = - a^4b^2 + \dfrac{3}{2}a^3b^3\\[1em]$

Hence, 2a³ - 3a²b x $-\dfrac{1}{2}$ ab² = -a⁴b² + $\dfrac{3}{2}$ a³b³