Calculate the value of expression [[$a^3 -6b $]] , when

a) [[$a = 2$]] and [[$b = 3$]].
b) [[$a = -1$]] and [[$b = 5$]].
c) [[$a = 3$]] and [[$b= -4$]].
d) [[$a = -4$]] and [[$b = -2$]].