So with a $600 difference on selling merchandise, there is a $132 difference in how much she earns. So basically, for every $100 she sells, she gets $22 in return. So if we multiply 22 by 23, we get 506, which is how much extra she is paid from selling $1000 to selling $3300. So just add 506 to 520, and you get $1026 as how much her salary is.
Hope this helped, hope this answer isn't too late! :)