Arithmetic as variable?

I have function in which perform arithmetic on standart numbers.

function arithmetic(formula)
	x = 1
	y = 10
	z = formula
	print(z)
end

arithmetic(x+y)

The problem is: it starts to count in function variables, when x and y are nil, and error happens because x and y are nil at the first line.

So i want to perform arithmetic only when i want to, in this case only on line 4.

x & y are not defined globally in the script in line 8. When you're calling a function, it takes in arguments for the function to play with.

function Arithmetic(x, y) -- Parameters for arguments
	local Solution = x + y
	print(Solution)
end
-- At this point, the function is defined; the output will spit out the solution when it is called.

Arithmetic(1, 10) -- Here, the defined function "Arithmetic" is called, taking 1 and 10 for x and y respectively.

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