>>61
a^2+7a*b+12*b^2+a+3b-9=6
(a+3b)(a+4b)+(a+3b)=15
(a+3b)(a+4b+1)=15

(a+3b,a+4b+1)=(-15,-1),(-5,-3),(-3,-5),(-1,-15),
          (15,1),(5,3),(3,5),(1,15)