Select the functions that satisfy the following differential equation:

$\frac{d^{2}y(x)}{d{x}^2}=ay(x)$

$y(x)=ax+b$

$C(x)=C_0{e}^{-kx}$

$(x)=cos(ax)+6$

$y(x)=-sin(3x)$

$y(x)=a{x}^2+bx+c$