You have a big misunderstanding about class and member methods.
What have you done is try to create a new TestClass with 6 parameters, where TestClass only has a default constructor, that have no parameters at all.
And what you have to do is creating the TestClass, than calling it's method...
TestClass t1 = new TestClass();
TestClass.ArithematicOperation(M1, M2, M3, M4, M5, out avg);