Factorise

$8 a^{3}+27 b^{3}+36 a^{2} b+54 a b^{2}=(2 a)^{3}+(3 b)^{3}+3(2 a)^{2}(3 b)+3(2 a)(3 b)^{2}$

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

Hence, factorisation of $8 a^{3}+27 b^{3}+36 a^{2} b+54 a b^{2}$ is $(2 a+3 b)^{3}$.