If all the signs are negative in an $(a + b + c)^2$ bracket, can I just make them all positive? I have to do the expansion
$$(-y - z - x^2 - y^2 - z^2)^2$$
Can I say that this is 
$$(y + z + x^2 + y^2 + z^2)^2$$
as all the signs are the same inside the brackets and so multiplying two negatives together will always give me a positive?
Or if I wanted to show it algebraically, I could do
$$(-y - z - x^2 - y^2 - z^2)^2 = [(-1)(y + z + x^2 + y^2 +z^2)]^2$$
$$ = (-1)^2(y + z + x^2 + y^2 +z^2)^2 = (y + z + x^2 + y^2 +z^2)^2$$
EDIT: Ok, lets say just one of those terms in that bracket was positive, could I still do the $(-1)$ trick and make just one term negative and so its easier to work out, or would I need to leave it as it is and expand it?

Yes.
$$(-a -b -c)^2 = ((-1)(a + b + c))^2 = \underbrace{(-1)^2}_{=+1}(a+b+c)^2 = (a+b+c)^2$$
