a)

$3\cdot \left(-21\right)=-63$

$2\cdot \left(-55\right)=-110$

$\left(-4\right)\cdot 12=-48$

$\left(-18\right)\cdot \left(-3\right)=54$

b)

$35:\left(-7\right)=-5$

$\left(-84\right):2=-42$

$\left(-90\right):\left(-30\right)=3$

$56:\left(-7\right)=-8$

c)

$\left(-2\right)\cdot 3\cdot \left(-9\right)=54$

$\left(-6\right)\cdot \left(-8\right)\cdot \left(-2\right)=-96$

$12\cdot \left(-3\right)\cdot 4=-144$

$20:\left(-5\right):\left(-2\right)=2$

d)

${\left(-8\right)}^2=64$

${\left(-2\right)}^3=-8$

${\left(-1\right)}^3=-1$

${\left(-6\right)}^2=36$