Answer
False
Work Step by Step
Given $D(D^2-4)^2y=0$
and $y(x)=c_1e^{-3x}+c_2xe^{-3x}+c_3\cos 5x+c_4\sin 5x$
Obtain: $D(D^2-4)^2y\\
=(D^5-8D^3+16D)y\\
=D^5y-8D^3y+16Dy$
Substituting: $0-0+16=16$
Hence, $y(x)$ is not a solution for the equation.