Momentum p = mv is conserved.
before: p = 2.5 * 7.5 + 70 * 0
after: p = (2.5 + 70) * v
p before = p after
2.5 * 7.5 = (2.5 + 70) * v
v = 2.5 * 7.0 / (2.5 + 70)
if i remember correctly, to get acceleration, you subtract the initial velocity from the final velocity and divide it by the time.
if you do that (11-7) / 3, it gives you 4/3 m/s^2
If time is specified, the distance may be estimated in constant acceleration using the formula: X=(at2)/2 if the beginning velocity is 0. (A automobile begins from a stop...) As a result, X=(6*10*10)/2=600/2 = 300 m.