To get a side length of 12,
The perimeter of triangle A is 3 + 4 + 6 = 13, so the possible perimeters of triangle B are:
12 + 16 + 24 = 52 (following an enlargement by a scale factor of 4)
or
9 + 12 + 18 = 39 (following an enlargement by a scale factor of 3)
or
and 6 + 8 + 12 = 26 (following an enlargement by a scale factor of 2)