Answer :
let A, B, C be angles of the triangle
A = 2B; so B = A/2
Therefore B is the smallest angle, so C = 4B + 12 = 2A + 12
now, A + B + C = 180, by angle sum property of a triangle
since we know that B = A/2 and C = 2A+12, lets substitute both of them in that equation.
A + A/2 + 2A + 12 = 180
(2A+A+4A+24)/2 = 180
7A + 24 = 180 * 2
A = (360-24)/7 = 48
Thus, B = A/2 = 48/2= 24 and C = 4(24)+12 = 108.