Create Python-Function-Hex.md

For issue freeCodeCamp#819

Python-Function-Hex.md

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+# Python hex(x)
+
+`hex(x)` is a built-in function in Python 3 to convert an integer number to a lowercase [hexadecimal](https://www.mathsisfun.com/hexadecimals.html) string prefixed with “0x”.
+
+## Argument
+
+This function takes one argument `x` that should be of integer type.
+
+## Return
+
+This function returns a lowercase hexadecimal string prefixed with "0x".
+
+## Example
+
+```python
+print hex(16)   # prints  0x10
+print hex(-298) # prints -0x12a
+print hex(543)  # prints  0x21f
+```
+
+:rocket: [Run Code](https://repl.it/CVCR)
+
+[Official Documentation](https://docs.python.org/3/library/functions.html#hex)