a) $2(a+b)^3 + 16 = 2[(a+b)^3+8]$
$= 2[(a+b)^3 + 2^3]$
$= 2[(a+b+2)[(a+b)^2 + 2^2 - 2(a+b)]]$
$= 2(a+b+2)(a^2 + b^2 + 2ab - 2a - 2b + 4)$
b) $8xy^3 + x(x-y)^3$
$= x[8y^3 + (x-y)^3]$
$= x[(2y)^2 + (x-y)^3]$
$= x(2y + x-y)(4y^2 + (x-y)^2 -2y(x-y))$
$= x(x+y)(x^2 -4xy +7y^2)$