Answer: 4mm
Explanation: From Hooke's law.
Tensile force is directly proportional to the elongation
That's
F= k e ， where k is constant
F/e = k
F1/e1 =F2/e2
Where e1 = 2mm
F2 =2F1 (doubled)
F1/2 = 2F1/e2
1/2 =2/e2
e2 = 4mm
Therefore when tension is doubled, the metal bar will be elongated by 4mm
