Multiply :

$-\dfrac{2}{3}$ a⁷b² and $-\dfrac{9}{4}$ ab⁵

$-\dfrac{2}{3}a^7b^2 \times \Big(-\dfrac{9}{4}ab^5\Big)\\[1em] = \dfrac{2 \times 9}{3 \times 4}a^{7 + 1}b^{2+5}\\[1em] = \dfrac{18}{12}a^8b^7\\[1em] = \dfrac{3}{2}a^8b^7\\[1em]$

Hence, - $\dfrac{2}{3}$ a⁷b² x - $\dfrac{9}{4}$ ab⁵ = $\dfrac{3}{2}$ a⁸b⁷