Scripts run in the workspace they are called from; that may be the base workspace, but in this case it's the workspace of the function ButtonPushed. Therefore, assigning variables to the base workspace is not needed. You can simply name the variables a_value and b_value instead of a and b. (Also, you'll need to convert the numeric result to text, and set the Text property of app.ResultLabel).
% Button pushed function: Button
function ButtonPushed(app, event)
a_value = app.Spinner.Value;
b_value = app.Spinner_2.Value;
Test;
app.ResultLabel.Text = num2str(c);
end
