This is what MathCad makes of your sum.
You are not summing from i = 1 to i = 107; you are summing from i = 29 to i = 107. As such, you should have only 107 - 29 + 1 = 81 terms. This makes the first summation that you broke out, the "four times the summation", equal to 4*81 = 324, not 4*107 = 428.
The second summation can be represented as three times the difference of the sum of the first 107 numbers and the sum of the first 28 numbers.
Apply the formula, do the subtraction, and multiply by 3. Then add in the "four times" part.