trouble with code to form robust solver that can handle different unknowns

I am trying to write some code that can identify an unknown and solve an equation with respect to it. For example, my input box asks for values for x,y,z, and w. At the moment if I enter values for everything except x it will find a solution for x and prompt the user, but it only works for x. How can I modify my code so it can solve for another variable without explicitly telling it what unknown it will be?

clear
prompt = {'x','y','z','w'};
dlg_title = 'Input';
num_lines = 1;
defaultans = {'...','...','...','...'};
options.Interpreter = 'tex';
answer = inputdlg(prompt,dlg_title,num_lines,defaultans,options)
test_for_numbers=str2double(answer);
if isnan(test_for_numbers)
       % str is a "symbol"
else
      % num is numeric value
end    
syms x
Solution = solve(x+test_for_numbers(2,1)+test_for_numbers(3,1)+test_for_numbers(4,1)== 100, x)
x = Solution 
CreateStruct.Interpreter = 'tex';
CreateStruct.WindowStyle = 'modal';
h=msgbox(sprintf('x = %2.3g',x),'x',CreateStruct);

Answer 1

[Walter Roberson]

You can use symvar() on a character string (even without the Symbolic toolbox.)