string input = "(5 + 9) please, send the solution to the arithmetic operation to proceed further. Thank you!";
int first = 0, second = 0, solution = 0;
var units = Regex.Matches(input, @"\d");
if (units.Count == 2)
{
    first = int.Parse(units[0].Value);
    second = int.Parse(units[1].Value);
    var op = Regex.Match(input, @"(?<=\d\s).*(?=\s\d)").Value;
    if (op == "+") solution = first + second;
    else if (op == "-") solution = first - second;
    else if (op == "*") solution = first * second;
    return solution.ToString();
}
else return null;