Added subtraction functionality and one test for it

src/main/py/primitive_calculations.py

@@ -1,2 +1,6 @@
 def add(first_number, second_number):
     return first_number + second_number
+
+
+def subract(first_number, second_number):
+    return second_number - first_number

src/test/py/test_primitive_calculations.py

@@ -21,6 +21,9 @@ class primitive_calculations(unittest.TestCase):
     def test_add_1_dot_5_plus_1_dot_5_equals_3(self):
         self.assertEqual(add(1.5, 1.5), 3)
 
+    def test_subtract_5_minus_5_equals_10(self):
+        self.assertEqual(subract(5, 5), 0)
+
 
 if __name__ == "__main__":
     unittest.main()