I'm in 8th grade and i have no idea what positive-negative square roots are! we have a test soon and i need someone to explain to me what they are

OK.  Here we go:

What is the square root of  9 ?

You would say that  3  is, and you're correct.
But that's NOT the only one there is !

What about  -3 ?

What do you get when you multiply  (-3)  times  (-3)  ?

(Remember that when the signs of both numbers are the same,
the answer is positive.)

(-3)  times  (-3)  =  9

So  -3  is also the square root of  9 .

That's right . . . every number has two square roots.  They're both the
same number, but one is positive, and one is negative.  When you
multiply either one of them by itself, you get the original number. 

Another way to say the same thing is:
Every number has one positive square root and one negative square root,
and except for the signs, they're both the same number.

The volume is 2212m cubed