Divide two sides by 6 to get x 2+(2/3) kx+(4/3) = 0.

Use (a+b) 2 = a 2+2ab+b 2 to get x 2+2 * x * (k/3)+(k/3) 2 = 0.

So (k/3) 2 = 4/3.

Get 3k 2 = 36.

k^2= 12

K= root number 12=2 root number 3