\[a^{3} + b^{3} - a^{2}b - ab^{2} =\]
\[= a^{3} - a^{2}b + b^{3} - ab^{2} =\]
\[= a^{2}(a - b) + b^{2}(b - a) =\]
\[= a^{2}(a - b) - b^{2}(a - b) =\]
\[= (a - b)\left( a^{2} - b^{2} \right) =\]
\[= (a - b)(a - b)(a + b)\]