2nd order linear equation solving for a, with boundary conditions.

Hello I'm having problem solving the following:

Let *a* be a real constant. Consider the equation

y''+y'+ay=0

]with boundary conditions *y*(0)=0 and *y*(6)=0.

For certain discrete values of *a*, this equation can have non-zero solutions. Find the three smallest values of *a* for which this is the case.

Enter your answers in increasing order, I need to find a_1, a_2, a_3

I've tried solving using the following Y(x) = C_1 e^((-3+3sqrt(1-4a))x)+C_2 e^((-3+3sqrt(1-4a))x), but i am not getting the right answer.