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