so, the first term in the arithmetic sequence is -1, then the fourth is -4, the tenth is -10... hmmmm
notice, an arithmetic sequence has a "additioner", or a "common difference", from -1 to 3 terms later it ends up as -4, so the common difference is -1.
so, therefore the sequence is then -1, (-1-1) = -2, (-2-1)=-3, (-3-1)= -4 and so on.
so f(-1) = -1, f(4) = -4, f(10) = -10, thus f(n) = -n