Answer:
0.66c
Explanation:
Use length contraction equation:
L = Lā ā(1 ā (v²/c²))
where L is the contracted length,
Lā is the length at 0 velocity,
v is the velocity,
and c is the speed of light.
900 = 1200 ā(1 ā (v²/c²))
3/4 = ā(1 ā (v²/c²))
9/16 = 1 ā (v²/c²)
v²/c² = 7/16
v = ¼ā7 c
v ā 0.66 c