Answer
(a) $ y''' = 0$
(b) $y''' = -72x^{-5} -24x^{-4}$
(c) $y''' = 24ax$
Work Step by Step
(a) $y' = 10x-4$
$\Rightarrow y'' = 10$
$\Rightarrow y''' = 0$
(b) $y' = -6x^{-3} -4x^{-2} + 1$
$\Rightarrow y'' = 18x^{-4} +8x^{-3}$
$\Rightarrow y''' = -72x^{-5} -24x^{-4}$
(c) $y' = 4ax^{3} +2bx$
$\Rightarrow y'' = 12ax^{2} + 2b$
$\Rightarrow y''' = 24ax$