Use Power Series to Solve the Differential Equation $$y ^ { tt } - x y ^ { t} - y = 0 , y ( 0 ) = 6 , y ^ {t } ( 0 ) = 0$$

Use power series to solve the differential equation.
$$y ^ { tt } - x y ^ { t} - y = 0 , y ( 0 ) = 6 , y ^ {t } ( 0 ) = 0$$