(b)
$y = ae^{x} + be^{- x} + c$
On differentiating w.r.t. $x$, we get
$y^{'} = ae^{x} - be^{- x}$
Again differentiating w.r.t. $x$, we get
$y^{''} = ae^{x} + be^{- x}$
Again differentiating w.r.t. $x$, we get
$y^{'''} = ae^{x} - be^{- x} = y'$