Ineed to explain how objects can have the same volume but different mass. ? ?

No problem, and you already know all about it.

Here are a few examples of same volume / different weight:

-- A bottle full of water is heavier than the same bottle when it's full of air.
-- Stones are heavier than styrofoam chunks the same size.
-- A bowl of meat loaf is heavier than a bowl of scrambled eggs.

In each example, two things have the same volume, but one weighs more than
the other.  I didn't say anything about mass yet, but that's easy:  As long as you
keep everything on Earth, more weight means more mass.

So how come, in each example, things with the same volume have different mass ?
This was your original question.

The answer is just the simple fact that there are millions of different substances, and
each different substance packs a different amount of mass into the same volume.

The amount of mass that a substance packs into a standard volume is called
the density of the substance.  Meat loaf is more dense than scrambled eggs.
Stone is more dense than styrofoam.  Water is more dense than air.  And gold
is 19 times as dense as water.  If you have a jar that holds a pound of water, and
you pour out the water and fill the jar with gold, the same jar holds 19 pounds of gold,
because the density of gold is 19 times the density of water.

The reason you were assigned to think about this question for homework is that
next time your Physics class meets, you'll start talking about Density. And you're
all ready for it now.

What formula  this one  (x_/

I'm not sure what your asking buddy.. but i know that you can weld two peices of steel by pure force and pressure.