A 139lb student lands from a vertical jump and immediately jumps vertically again. The average ground reaction force is 1345N which is applied for .3s. He was moving at 1.1m/s downward when he first contacted the ground. What is his takeoff velocity?

I know that I need to determine the resultant force in N, but I am having trouble figuring out how to do that. Then, I know I can use the impulse/momentum equation. Please help. Thanks!

You're told what the reaction force is - it's 1345 N, so no need to calculate it. Use F=ma to figure out the acceleration, and then since you know the duration of the force being applied you can calculate the change in velocity. Be sure to convert the student's mass from pounds to Kg (weird that the problem mixes imperial and SI units like that - are you in Britain?). Hope this helps.