maths a level mei core 2 sequences and series question URGENT HELP

The first term of an arithmetic sequence is 120 and the common difference is -7
i) for what value of k is the kth term first negative
ii) for what value of n is the sum of the first n terms first negative
how the hell do I answer these? Please help!

for i) use the formula for nth term,

$U_n = a + (n-1)d$

use n = k, and rearrange the equation to find k when $U_k$ is less than 0, in other words let $U_k = 0$ and find the value of k. round up as k is an integer.

for ii) use the sum of n terms formula,

$S_n = \frac{1}{2}n(2a + (n-1)d)$

Its the same thing, find the value of n when $S_n$ is less than 0. This will be a quadratic so you ignore the answer that gives the negative value

Ok, I understand the first part now, thanks a million, but still unsure about how to do the last part...