For which values of M is the expression factorable. Show or explain your logic:
x^2+Mx+12
pick two factors of 12 with the same sign. So,
(x-1)(x-12) = x^2 - 13x + 12
(x+1)(x+12)
...
(x+3)(x+4) = x^2 + 7x + 12
fill in the others possibilities using factors of 12