A gorilla is hanging motionless on a tree branch. If the gorilla's weight is 400 Newtons, what must be the tension in the tree branch?

let's convert 400 newtons to Kg, which is 40.7886.

now we put it in a formula that includes gravity and acceleration

(40.7886 kg) (9.8 m/s2) + (40.7886 kg) (0)

now we come up with the answer of 399.72828 kg or 3920 Newtons

Not sure which unit you are looking for but they are both there.