The graph of f (x) = 2x + 4 shifts six units to the right when it is replaced with the graph of f (x) = 2x - k. What is the value of k?

I believe that it should be 2.

+4 means it is shifted 4 units to the left from the y axis. And - 2 means it shifts 2 units to the right from the y axis. So the shift of four would leave it at - 4 for the x axis and the shift of four would leave you at positive 2 for the x axis. The difference of - 4 and two is six, proving that your answer is 2.

Sorry if this is wrong or doesn't make sense.