What is the momentum of the child and the skateboard?

Question

A 21 kg child is riding a 5.9 kg bike with a velocity of 4.5 m/s to the northwest.

Answer 1

you can make a triangle to find the linear component of the velocity which I got as 4.5cos45=3.18m/s

p=mv=(21+5.9) x 3.18

p=85.6kgm/s

hope thats right

Answer 2

Why use the horizontal component of velocity?

I would say 4.5(21+ 5) as the momentum, since you want the whole velocity to be included.