Hello i need help (calculator)

I did try this but what i need is VariableString(_firstNumber)VariableString(_secondNumber)*VariableString(_thirdNumber)

is this type of thing possible because when i did try it is says taht i need to put a sign in between the first and second but in the formula that i need to use the first and second number should only be next to each other

[edit] If this is not possible is there any other way I can make it work? I need this for my project and this is the only thing that I need to do to complete it