CPM Homework Help : A2C Problem 4-153

If $x^2 + kx + 18$ is factorable, what are the possible values of $k$ ?

Hint:

2 by 2 square, labeled as follows: top edge, left, x, left edge, top, x, interior, top left, x squared, interior, bottom right, 18.

${\begin{array} {c c c} \ \ \ 1&\ \ \ 2&\ \ \ 3&\ \ \ 6&\ \ \ 9&\ \ \ 18\\ -1&-2&-3&-6&-9&-18\\ \quad \ 1x&\quad \ 2x&\quad \ 3x&\quad \ 6x&\quad \ 9x&\quad \ 18x\\ \ -1x&\ -2x&\ -3x&\ -6x&\ -9x&\ -18x \end{array}}$

Find expressions that will complete the generic rectangle.

Answer:

There are 6 answers: $+19$ and $-19$ are two of them.
Find the other 4 answers.