Calculate: a^3+27

Expression: $a^{3}+27$

Rewrite $27$ as $3^{3}$

$=a^{3}+3^{3}$

Apply Sum of Cubes Formula: $x^{3}+y^{3}=(x+y)(x^{2}-xy+y^{2})$

$=(a+3)(a^{2}-3a+3^{2})$

$3^{2}=9$

$=(a+3)(a^{2}-3a+9)$