I'm happy to do the psudo-code for you as this program is really simple.Although I won't be writing the code. I suggest you write some code following the below steps and once you get stuck, just raise another quetion with the code you've achieved.

make these variables: 1-sum (perhaps global but it doesn't matter in this case) 2-min_num ( it takes in user input)
**3-max_num **(it takes in user input)

then step 4 -> do some checking to see the variables (min_num & max_num) are not empty and contain some values. You'll need to parse them as integers or whatever type you're expecting from your users such as float. It might be a good idea to display a message if users are trying to use your calculator but they aren't entering anything or entering some text which is not acceptable.

5- Use an if-statement to compare min_num < max_num

6- inside your if-statement block.. do your formula. Here where you use the sum to store the result in then return it to users. And if you want to go a little nuts with the result you can say if sum <= 0, then you can return a message to your users saying.. The total is less than or equal to zero.

And I should get 14 but I get 0 why?
I thing I have to use forloop to get the total Number of intgers
total of integer 2, 3, 4, 5 equal, 1/24(2 + 5) = 2*7 = 14.

`if (min_num < max_num)
{
Number of intgers= min + max;
sum = 1 / 2 * Number of intgers * (min_num + max_num);
}
txtSum.Text = sum.ToString();`

So I am having trouble knowing what to return in the bottom method any help would be appreciated .
Here are the specifications for completing the StringArray class: