Find the function $f(t)$ of the form $f(t)=a \cos (2 t)+$ $b \sin (2 t)$ such that $f^{\prime \prime}(t)+2 f^{\prime}(t)+3 f(t)=17 \cos (2 t)$
(This is the kind of differential equation you might have to solve when dealing with forced damped oscillators, in physics or engineering.)