define an equation and get the variable value after setting the derivative to zero

hello

im trying to define a function then derive it and set the derivative to zero to obtain a number then plug the number to the function to get an answer.

i keep getting an error:

Error using symengine
The dimensions do not match.

Error in sym/privBinaryOp (line 937)
            Csym = mupadmex(op,args{1}.s, args{2}.s, varargin{:});

Error in  *  (line 270)
        X = privBinaryOp(A, B, 'symobj::mtimes');

Error in Project (line 32)
        w(i)=(2*Mp/(l(i)^2))*((2-(solx/l(i)))/((solx/l(i))*(1-(solx/l(i)))));

the code is listed below

clear all;
clc

disp('Answer the following');
b=zeros(1,1);
h=zeros(1,1);
l=zeros(1,4);
YStress=zeros(1,1);
Mp=zeros(1,1);
w=zeros(1,4);

for i=1:1
    b(i)=input('Enter the width of this span in inches   ');
    h(i)=input('Enter the depth of this span in inches   ');
    YStress(i)=input('Enter the yield stress of this span in ksi   ');
    Mp(i)=(b*((h/2)^2)*YStress)/12;
end

for i=1:4
    disp('Span');
    disp(i);
    disp(' ');
    l(i)=input('Enter the length of this span in feet   ');
end

for i=1:4
    if i==1
        syms f(x);
        f(x)=(2*Mp/(l(i)^2))*((2-(x/l(i)))/((x/l(i))*(1-(x/l(i)))));
        df=diff(f(x),x);
        solx=solve(df==0,x);
        w(i)=(2*Mp/(l(i)^2))*((2-(solx/l(i)))/((solx/l(i))*(1-(solx/l(i)))));
    elseif i==4
        syms f(x);
        f(x)=(2*Mp/(l(i)^2))*((2-(x/l(i)))/((x/l(i))*(1-(x/l(i)))));
        df=diff(f(x),x);
        eqn=df==0;
        solx=solve(df==0,x);
        w(i)=(2*Mp/(l(i)^2))*((2-(solx/l(i)))/((solx/l(i))*(1-(solx/l(i)))));
    else
        w(i)=(8*Mp)/(l(i)^2);
    end
end

Any help is appreciated