Recall that
[tex]v_f=v_i+at[/tex]
which follows from the definition for average acceleration. So Lucifer has a final velocity of
[tex]5.48\dfrac{\rm m}{\rm s}+\left(0.892\dfrac{\rm m}{\mathrm s^2}\right)(6.90\,\mathrm s)\approx\boxed{11.6\dfrac{\rm m}{\rm s}}[/tex]
