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!