Solve
0=e^3x-9e^2x+20e^x
You may find this easier if you let u = e^x. Then you have
u^3-9u^2+20u = 0
u(u^2-9u+20) = 0
I think you can probably factor that.
Of course, one solution is
u=0, or e^x = 0
But that never happens, so you can throw it out.