Introducing XY=5
=(6*5+7y)+8x-(5*5-y+6x)
=30+7y+(8x-25+y-6x)
=5+8y+2x
=5+2(x+4y)
(x+4y)=- 1 brought in
=5-2
=3
Method 2:
=(6*5+7y)+8x-(5*5-y+6x)
=30+7y+(8x-25+y-6x)
=5+8y+2x
Because x+4y=- 1, x=- 1-4y is brought in.
=5+8y+2(- 1-4y)
=5+8y-2-8y
=5-2
=3