Let ta be the time the first rocket travels and tb be the time the second rocket travels. ta = 2tb
We apply:
sa = ut + 1/2 at², with u = 0 since they start from rest.
310000 = 1/2a(ta)²
a = 620000 / 4(tb)²
sb = 1/2 a(tb)²
sb = 1/2 x 620000 / 4(tb)² x (tb)²
sb = 77.5 Km
We apply v = u + at, with u = 0
va = a(ta)
a = 320/2(tb)
vb = a(tb)
vb = (320/2tb)(tb)
vb = 160