Examples is (2x+3y+4)×(5x+4y+8)
The first number or the (2) will multiplied to the 3 number opposite to there group example:
(2x)×(5x)=10x to the second power(^2)
in organize form is
[10x^2) + 8xy + 16x]

and the next is the second number the number 3
(3y)×(4y)=12y to the second power(^2)
and in organize is
[12y^2) + 15xy + 24]

and the last one is the number 4
and in organize is
[20x + 24y + 16]

Then the last is to add all the same equation and the answer is
=10x^2 + 12y^2 + 23xy + 36x + 40y + 24

I hope it help you