added else case for no 'x' in function for difference_of_simplified_exponential_functions

src/main/py/read_math_function.py

@@ -131,3 +131,5 @@ def difference_of_simplified_exponential_functions(type, function):
     if new_minuend != "Der Ausdruck muss die Variable 'x' enthalten." and new_subtrahend != "Der Ausdruck muss die Variable 'x' enthalten.":
         simplified = new_minuend - new_subtrahend
         return simplified
+    else:
+        return "Der Ausdruck muss die Variable 'x' enthalten."