Answer:
Total momentum before collision
P1 =.4 * 3.5 = 1.4 ignoring units here
Total momentum after collision
P2 = .6 * V - .4 * .7 = .6 V - .28
.6 V = 1.4 + .28 momentum before = momentum after
V = 2.8 cm/sec
In 5 sec V moves 2.8 cm/sec * 5 sec = 14 cm