I suppose on that basis it could also be 12- the difference between the numbers on the right fall reduces by 2 each time.
But the 9 and 12 solutions ignore the pattern of the numbers on the left. But maybe that doesn't matter.
I wonder if donny has set it out exactly as it should be shown. Is there a line of dots as I showed it.
Anyway, I'm off to my maths class in a minute. I must make sure I have a pile of black pens and rulers with me. Oh and some red pens to hand out for self marking and purple pens for peer assessemnt comments...