VB excel functions #Value!, making sub into a function

i have financial equation:
FV = (PV * (1 + (x / m)) ^ (m * t) + PMT * ((1 + (x / m)) ^ (m * t) - 1) / (x / m))
i need to express x from equation. I wrote sub which solves the problem and works perfectly but what i want is to use that as function (typing in cell =F() and get instant result). So i wrote function based on my sub and it doesnt work, every time i get #Value! error in cell. Why? can anyone help me.
as you will notice i post both my sub and function(wich doesn't word) in one code, just sub syntax and variables is made as text notifications.
My code:

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