I think mult just works on the vector
position.mult(-1);

it doesn't return anything (only void)

so you want

1. PVector reverse = position.get();
2. reverse.mult(-1);

Greetings, Chrisir

If you need an answer, please send me a personal message since this forum doesn't notify.

1. PVector position=new PVector(1, 1, 1);
2. PVector reverse = position.get();
3. reverse.mult(-1);
4. println(reverse);

It's a pity those methods return just a void. It'd be gr8 if they returned this. So we could chain them together. 

there's another form of all the PVector calls that return a new PVector containing the result. just call it using the static PVector and pass the original PVector in as the first argument.

(it's in the reference but it's not clear)

PVector v2 = PVector.mult(v1, 6);

returns a new PVector that is 6 times the one passed in.