Rewriting Particular soltution

[polarbear123]

Hello,
I have found a particular solution int he form of
y=Pcos(ax)+Qsin(ax)
but have to find a particular in the form of
y=Re(Ae^ix)
How can I rewrite, or change the equation so the first is the latter?

 

[HallsofIvy]

$\displaystyle cos(ax)= \frac{e^{iax}+ e^{-iax}}{2}$
$\displaystyle sin(ax)= \frac{e^{iax}- e^{-iax}}{2i}$