Because you're trying to return a Float: tagged variable through a non-tagged stock.
Code:
stock calculation(const Float:x)
{
new Float:Result
if(x > 1)
Result = 3 / (x - 1)
else if(x == 1)
Result = x * 0
else if(x < 1)
Result = (4 * x * x) - 2
return Result
}
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