The sum of the first `n` natural numbers (`1 + 2 + 3 + ... + n`) is given by the formula:

`<sup>1</sup>/<sub>2</sub>n(n + 1)`

Grace starts to sum up the numbers from `11` as follows:

`11 + 12 + 13 + ... + n = 3185`

Show that `n<sup>2</sup> + n − 6480 = 0`.

Find the value of `n`.