Calculating Velocity after Skateboard Jump - A Physics Problem

[adriannesmith]

Homework Statement

Dan is gliding on his skateboard at 3.00m/s . He suddenly jumps backward off the skateboard, kicking the skateboard forward at 6.00m/s (as measured by an observer on the ground). Dan's mass is 40.0kg and the skateboard's mass is 5.00kg . How fast is Dan going as his feet hit the ground?

Homework Equations

vf-vi=vexplosion*ln(mf/mi)

The Attempt at a Solution

vi=3 m/s
mi=45 kg
mf of dan=40 kg
I used 6 m/s as vex
when I plugged the above in the equation to find vf I got 2.29 which was marked wrong. Is it wrong to use 6m/s for Vex?

 

[adriannesmith]

The correct answer is 2.63 m/s, but I'm still confused on the process to get the answer