The cost of notebook = "x"
The cost of the pen = "2y"
(y x 2 because it is twice of the notebook)

=> x - 2y=0

hope helped you

1 5 1
The cost of the pen = "y"
but you have written cost of pen = 2y
o ya right..that has to be ;-
cost of notebook=2 x cost of pen(y)
=> x - 2y = 0

is this right answer!! ;D
as i have edited my mistake so please do not report or delete my answer