What, exactly did the problem say? What you've written here makes no sense.

In the sequence 200, 5000, 200 is the first term , 5000 is the last term and there are 2 terms. The sum is 5200 or, using your formula, (2/2)(200+ 5000)= 5200.

In the sequence 200, 2600, 5000, 200 is the first term, 5000 is the last term and there are 3 terms. The sum is 7800 or, using your formula, (3/2)(200+ 5000)= (3/2)(5200)= 3(2600)= 7800.

You cannot find either the number of terms nor the sum of an arithmetic series just from the first and last numbers. You have to know what the series is!