To solve this equation, we first need to find the sum of the arithmetic series from x to x+505.

The sum of an arithmetic series can be calculated using the formula:

S = n/2 * (first term + last term)

In this case, the first term is x and the last term is x+505. The number of terms in the series is 506 (from x to x+505).

So, the sum of the series is:

S = 506/2 * (x + (x+505))
S = 253 * (2x + 505)
S = 253 * 2x + 253 * 505
S = 506x + 127765

Now, we can set this sum equal to 129789 and solve for x:

506x + 127765 = 129789
506x = 2024
x = 4

Therefore, the value of x is 4.