just use the co-efficient to root relationships
1) let the roots be A and 2A
A+2A=12
A=4
2A=8
c=2A^2
c=32
2) x^2 + bx + 36 = 0, integral roots
let the roots be A and B
36= A x B
(A,B)=(4,9)(-4,-9)(3,12)(-3,-12)(6,6)(-6,-6)(2,18)(-2,-18)
Notice that (A,B) is the same as (B,A) [FYI]
So...