Solutions of a linear, second-order, homogeneous differential equation

[Holly1990]

Homework Statement
Show, by means of the Wronskian, that a linear, second-order, homogeneous diffferential equation of the form y''+P(x)y'+Q(x)y=0 cannot have 3 independent solutions.

The attempt at a solution
I tried by constructing a Wronskian of 3 general solutions expecting the wronskian to disappear but it didn't so I'm guessing I've gone about this the wrong way?

 

[vela]

Let y₁, y₂, and y₃ be solutions to the differential equation. That means

y₁'' + P(x) y₁' + Q(x) y₁ = 0
y₂'' + P(x) y₂' + Q(x) y₂ = 0
y₃'' + P(x) y₃' + Q(x) y₃ = 0

Try expressing that system of equations in matrix form.