Read a function in a GUI

Hello. I'm working with a GUI and i don't how to introduce a function to be evaluate in my program. I started to use syms like a synonymous of inline, but i'm having problems with it. I hope you can help me. Note: I'm trying to solve a bisection method.

function BtnCalcular_Callback(hObject, eventdata, handles)

syms x; 
Funcion=get(handles.EtxtFuncion,'String');
A=str2double(get(handles.EtxtInterA,'String'));
B=str2double(get(handles.EtxtInterB,'String'));
Tol=str2double(get(handles.EtxtTol,'String'));
e=100;

while e>=Tol
    
    Xr=(A+B)/2;
    
    e=Xr-A;
    
    fXr=subs(Funcion,Xr); 
    fXa=subs(Funcion,A); 
    
    sig=(fXr)*(fXa);
    
    if sig>=0
        A=Xr;
    else
        B=Xr;
    end
  
end

set(handles.ResRaiz,'String',num2str(A));

Answer 1

[Walter Roberson]

    fXr = double(subs(Funcion,x,Xr)); 
    fXa = double(subs(Funcion,x,A)); 

[Alejandra Chávez]

Thank you so much :)