If you want to do "z:"+cta then "y:90d",
you should set the property to "z:"+cta+" & x:90d"
If you want to do "y:90d" then "z:"+cta
you should set the property to "x:90d & z:"+cta

Because cta is a variable so it need to be outside the string and added to property string with "+" operator.