$250 - in his account
$35 per month out of (minus $35 per month from his account)
$250 - 1 · $35 - after the first month
$250 - 2 · $35 - after the second month
$250 - 3 · $35 - after the third month
...
n - the number of a month
The function:
f(n) = 250 - 35n
Domain:
250 - 35n > 0 subtract 250 from both sides
-35n > -250 change the signs
35n < 250 divide both sides by 35
n < 7.142.... → n < 8 → n ∈ {0, 1, 2, 3, 4, 5, 6, 7}