"Cannot access static method in non-static context" : I've just recently started my intermediate course in C# programming and I'm learning how to create multiple classes and creating methods to use in my program. It's a very new topic to me so I apologize if it's something very obvious or stupid. I keep getting the message "Cannot access static method in non-static context" in all of my methods.
Code in method class:

    public static int Add(params int[] numbers) {
            var sum = 0;

            foreach (var n in numbers) {
                sum += n;
            }

            return sum;
        }

        public static int Subtract(params int[] numbers) {
            var sum = 0;

            foreach (var n in numbers) {
                sum -= n;
            }

            return sum;
        }

        public static int Multiply(params int[] numbers) {
            var sum = 0;

            foreach (var n in numbers) {
                sum *= n;
            }

            return sum;
        }

        public static int Divide(params int[] numbers) {
            var sum = 0;

            foreach (var n in numbers) {
                sum /= n;
            }

            return sum;
        }

        public static string[] CheckingOfSomeSort(string userInput, int value, bool isAddition, bool isSubtraction, bool isDivision, bool isMultiplication) {
            if (userInput.Contains("+")) {
                var addition = userInput.Split('+');
                value = 1;
                isAddition = true;
                return addition;
            } else if (userInput.Contains("-")) {
                var subtraction = userInput.Split('-');
                value = 2;
                isSubtraction = true;
                return subtraction;
            } else if (userInput.Contains("*")) {
                var multiplication = userInput.Split('*');
                value = 3;
                isMultiplication = true;
                return multiplication;
            } else if (userInput.Contains("/")) {
                var division = userInput.Split('/');
                value = 4;
                isDivision = true;
                return division;
            }

            return null;
        }
I am attempting to create a calculator (which I have already done, however I'm trying to use methods now)