system of 1st and 2nd order differential equations ode45

I have a system of two 1st order diff eq and one 2nd order diff eq. I want to solve them simultaneously using ode45 and subsequently sum the solutions. I'm having troubles formulating the code for the 2nd order equation. Could you help? The code is:

Fx=@(t,xqxcxm)[ (-xqxcxm(1)+G_q .*(q-q_b)./q_b)./tau_q ;               
                                (-xqxcxm(2) +0.3+3.*tanh(Pa_co2./Pa_co2_b -1.1))./tau_co2 ;
                                  %insert 2nd order diff:    xm'' = (-tau2* xm' -xm)/tau1^2  ]

[t, xqxcxm]=ode45(F,[0 100], [0 0 0]);
xq= xqxcxm(:,1);
xc= xqxcxm(:,2);
xm= xqxcxm(:,3);

x=xm+ xc -xq;

[Walter Roberson]

It is our policy that we do not remove questions with a relevant Answer, not unless the question was abusive.

[gorilla3]

Could you please help me with this? It's still part of the question:

https://uk.mathworks.com/matlabcentral/answers/362979-ode-45-formulation-of-equations-in-system

Answer 1

[Josh Meyer]

The documentation explains how to express higher-order equations as first-order equations: Higher-Order ODEs

[gorilla3]

Hello,

here is a better definition of my question. Could you please help me? I believe I formulated one of the equations erroneously.

https://uk.mathworks.com/matlabcentral/answers/362979-ode-45-formulation-of-equations-in-system