[tex]\dfrac{3a}4\cdot\left(-\dfrac{2a}3\right)=\dfrac{3}4\cdot\left(-\dfrac{2}3\right)\cdot a\cdot a=-\dfrac24a^2=-\dfrac12a^2\quad \left\{=-\dfrac{a^2}2\right\}[/tex]
[tex]-2x\cdot(-3x)\cdot\dfrac14=-2\cdot(-3)\cdot\dfrac14\cdot x \cdot x=3\cdot\dfrac12\cdot x^2=\dfrac32x^2=1\dfrac12\,x^2[/tex]
[tex]4\cdot\left(-\dfrac{x}2\right)\cdot\left(-\dfrac{2x}4\right)= 4\cdot\left(-\dfrac12\right)\cdot\left(-\dfrac{2}4\right)\cdot x\cdot x= \dfrac12\cdot\dfrac{2}1\cdot x\cdot x=x^2[/tex]
[tex]-\dfrac{10m}7\cdot\dfrac{14m}{15}=-\dfrac{10}7\cdot\dfrac{14}{15}\cdot m\cdot m=-\dfrac{2}7\cdot\dfrac{14}{3}\cdot m^2=-\dfrac{2}1\cdot\dfrac{2}{3}\cdot m^2=-\dfrac4{3} m^2=-1\dfrac13\, m^2[/tex]