#(a - b)^3#
#(a - b)^3 = (a - b) (a - b) (a - b)#
#(a - b)^3 = (a - b) [(a - b) (a - b)]#
#(a - b)^3 = (a - b) [a^2 -ab -ab + b^2]#
#(a - b)^3 = (a - b) [a^2 -2ab + b^2]#
#(a - b)^3 = a (a^2 -2ab + b^2) - b (a^2 -2ab + b^2)#
#(a - b)^3 = a^3 -2a^2b + ab^2 - a^2b + 2ab^2 - b^3#
#(a - b)^3 = (a^3 - b^3) + (- 2a^2b - a^2b) + (ab^2 + 2ab^2)#
#(a - b)^3 = (a^3 - b^3) + (- 3a^2b) + (3ab^2)#
#(a - b)^3 = a^3 - b^3 - 3a^2b + 3ab^2#