\[\boxed{\mathbf{84}.}\]
\[\textbf{а)}\ y^{'} = 5y;\ \ y = e^{5x};\]
\[y = e^{5x} + C.\]
\[\textbf{б)}\ y^{''} = 25y;\ \ y = e^{5x};\]
\[y = Ce^{5x}.\]
\[\textbf{в)}\ y^{''} = 16y;\ \ y = e^{- 4x};\]
\[y = Ce^{- 4x}.\]
\[\textbf{г)}\ y^{''} = - 9y;\ \ y = \sin(3x + 5)\]