solution of equation based on initial guess

Hi everyone, I am trying to find the right value of SI, which gives me the equatilty betwenn Q_in and Q_out. I am using this approach:

%Input data
P=2.9;
Q_in=2.11;
P5=1.08;
lambda=0.2;
Fc=0;
SI=1.44; %initial guess

M=(P5-0.2*SI)*SI/(P5+0.8*SI);
if M<0
    M=0;
end
S=SI-M;
if S<0
    S=0;
end
Ia=lambda*S;
Q_out=(P-Ia-Fc).*(P-Ia-Fc+M)/(P-Ia-Fc+M+S);

Q_in-round(Q_out,5);

if (Q_in-round(Q_out,5))<1e-2
    a=round(Q_out,5)
end

Everything is depending on the tol, but when I increase it, I do not get the answer. Can someone help me?

[Walter Roberson]

You have

Q_in-round(Q_out,5);

That line computes a result, and then throws it away because of the semi-colon at the end of the line that says not to display output. You are not assigning the result to a variable, and you are not displaying the result.

[Torsten]

Use MATLAB's "fzero" to solve

Q_in - Q_out(SI) = 0