It's hard to explain, but theres a trick to it too.
Answer:
||w|| = 6
Step-by-step explanation:
||w|| = modulus or magnitude of vector w
Since formula to get the modulus of any vector = 
Vector w = <-6, 0>
x-component of vector w = (-6)
y-component of vector w = 0
Therefore, ||w|| = 
= 6
Sent a picture of the solution.
Im guessing that by making 10 it will speed up the addition process so it should be
B since (6+4) = 10 then +5 =15