a)
$= 8x^3 - 36x^2 + 54x - 27 - (8x^3 + 12x^2 + 6x + 1)$
$= 8x^3 - 36x^2 + 54x - 27 - 8x^3 - 12x^2 - 6x - 1$
$= -48x^2 + 48x - 28$
b)
$= 8x^3 - 36x^2y + 54xy^2 - 27y^3 + 8x^3 + 12x^2y + 6xy^2 + y^3$
$= 16x^3 - 24x^2y + 60xy^2 - 26y^3$
c)
$= 1/4x^2 - 3xy + 9y^2 - (4x^2 + 6xy + 9/4y^2)$
$= 1/4x^2 - 3xy + 9y^2 - 4x^2 - 6xy - 9/4y^2$
$= -15/4x^2 - 9xy + 27/4y^2$