volume of the parallelepiped

I have the three building vectors of two different parallelepipeds.

P = e1, e2, e3

G = ae3+e1, be3+e2, e3

I have to prove that

|(e1Xe2).e3| = |((ae3+e1)X(be3+e2)).e3|

where X vector product and
. scalar product

thanks.

see this :Volume of a Parallelepiped - YouTube