I have a friend Robert who is an expert at taking the difference of consecutive perfect squares. This means that if I give him a number $x$, he immediately computes $$x^2 - (x - 1)^2$$and gives me the result. I decided one day to first give Robert the number $9$, then the number $6$, and finally the number $4$. Robert gave me three numbers in return. What is the sum of these three numbers?