First solve \(\displaystyle y_h ^{ \prime \prime } - y_h = 0\) (The homogeneous solution.)

Then use variation of parameters for the particular solution. I'd start with \(\displaystyle y_p = Ax^2 e^x + B x e^x\)
(You could use \(\displaystyle y_p = A x^2 e^x + B x e^x + C e^x\) but the C term cancels out anyway.)