I assumed that "y" must not be a constant since you have dy/dt. therefore I made it an input for the function
% define your constans here:
Vs =
H1 =
Fd =
CT =
K1 =
H1 =
CD1 =
CD2 =
% function definition:
dydt = @(Q,Cin,y) Q.*(Cin-y)-(Vs/H1)*(1-Fd)*CT-(K1/H1)*(CD1-CD2)
if you want to call(calculate) the function for a specific set of inputs, simply use the syntax:
% define input arguments:
Q =
Cin =
y =
% call function
dydt(Q,Cin,y)
% notice that if you varaibles depend on time or any other varibles, they must be defined
% as functions too, like:
% Q = @(t) 1+t.^2
