Finding an unknown magnitude

Question

This is going to sound very weird, but how do you find a magnitude with unknown vectors? I'm currently stuck on a question that says "If vectors A and B are opposite vectors and the magnitude of A is 5, what is the magnitude of B?" I'm only looking for a way to answer the question and it's probably very simple, I just don't have anything related to the question in my notes.

Answer 1

Guide:

The key word is opposite vector.

That is we have the relationship $B=-A$. Hence given $\|A\|$, we should be able to find $\|B\|$.

Answer 2

Let $A=(a,b,c)$ then $-A=(-a,-b,-c)$.

We know that $$\sqrt {a^2+b^2+c^2 }=5$$ what is $$\sqrt {(-a)^2+(-b)^2+(-c)^2 }=?$$