In class 11th in the chapter of mole concept of chemistry , we study a formula of vapour density which is :-

Vapor density = (molecular weight) / 2

Now the formula mass of CO is = 28

Vapour density of the gas = 70

Therefore, by using the above formula -
Molecular weight of gas = 702 = 140

And we have to find x
So, x = 140/28 = 5

So the value of x in the above question is 5