what multiplies into -168 that can ad up to 13
-168
84 , - 2 sum= 82
42 , - 4 sum = 38
21 , - 8 sum = 13
x + (13-x) = -168
-x^2 +13 x + 168 = 0
x^2 -13x -168 = 0 Now use Quadratic formula to find x = 21 and -8
+37093 
