I'm teaching myself Delta Functions and I'm stuck on this initial value problem. If you guys can help me out, I'd appreciate it!

Solve the initial value problem : $\displaystyle x^{//}+9x=\delta (t-3\pi)+cos(3t); x(0)=x^{/}(0)=0$.

Also, I don't get how to do this problem:

Apply Duhamel's principle to write an integral formula for the solution of the initial value problem $\displaystyle x^{//}+4x^{/}+8x=f(t); x(0)=x^{/}(0)=0$