How to write function for my DAE system

Hi,

I have my below system need to be solve but I dont know how to write a function to solve it. I tried ode15s and solve for F( dx1dt, x2, dx3dt. dx4dt, dx5dt, dx6dt).

Thanks in advance!

dX1dt = (-G/(vo*dg)).*(D*X1) + ((kg*av*Mm*pt).*(X2-X1))./(vo*dg);
X2   =  -((dp).*f)./(12*kg); 
dX3dt = (-G/(vo*dg)).*(D*X3) + ((hf*av).*(X4-X3))./(vo*dg*Cpg); 
dX4dt = (6*hf.*(X3-X4))./(dp*Cps*ds)+(6.*(-Hh2).*kg.*X1)./(dp*Cps*ds)+((-Hni).*f)./(2*Cps*ds);  
dX5dt = -f./ds;   
dX6dt = ((-12.*kg).*X1)./(dp*ds);