The binomial coefficients for this are 1, 12, 66, 220, 495, 792, 924, 792, 495, 220, 66, 12, 1, so 924 is the middle coefficient: 924*(x^2)^6*3^6=924*729x^12=673596x^12.
(The coefficients are 12, 12*11/2=66, 12*11*10/(2*3)=220, 12*11*10*9/(2*3*4)=495, etc.)