What function do I use instead of "matlabFunction"?

It runs for a very long time and I need this function to use for define a function in my equation.

I want to define a function without use "matlabFunction". What function do i use to define a function?

Thank for help.

Example code

% Riemann summation loop
n2 = 1
dx1 = (TopR1-DownR1)/n2;
dy1 = (TopR2-DownR2)/n2;
dz1 = (TopR3-DownR3)/n2;
sumx = 0;
sumy = 0;
myequation = 0;
for i1 = 1:n2
    sumx=sumx+subs(U2,x1,dx1*rand+(DownR1+(i1-1)*dx1));     % U2 is a function have 4 variables
end
sumx = sumx*dx1;
for j1 = 1:n2
    sumy=sumy+subs(sumx,y1,dy1*rand+(DownR2+(j1-1)*dy1));
end
sumy = sumy*dy1;
for k1 = 1:n2
    myequation=myequation+subs(sumy,z1,dz1*rand+(DownR3+(k1-1)*dz1));
end 
myequation =myequation*dz1;
% plot graph
F1 = matlabFunction(myequation);
% range of varaible d
d = 0:0.01*(10^-9):20*(10^-9);
Potential = zeros(length(d));
for i1 =  1:length(d) 
        Potential(i1) = F1(d(i1));
end
figure('Name','Potential','NumberTitle','off');
plot(d,Potential)

[madhan ravi]

upload "myequation"

[madhan ravi]

Also I don't see why your using a loop.

[chayanon summueang]

I upload "myequation". It is a loop after Riemann summation and U2 is a function that have 4 variables.

*4 variables. Sorry