Let $y^{\,\prime\prime} - 3 y^{\,\prime} - 18 y = 0$.

1. Try a solution of the form $y = e^{rx}$, for some unknown constant $r$, by substituting it into the differential equation.
   $= 0$. help (formulas)
   
   
2. Simplify and factor your equation in the previous part as much as possible. Be sure to divide out any factors guaranteed to be nonzero.
   $= 0$. help (formulas)
   
   
3. Find all values of $r$ such that $y = e^{rx}$ satisfies the differential equation. If there is more than one correct answer, enter your answers as a comma separated list.
   $r =$ help (numbers)