Find relation between the following numbers with use Elementary arithmetic

Find relation between the following numbers:

we can use only Elementary arithmetic (addition, subtraction, multiplication, and division) . I find this puzzle in my old book .I think we have Arithmetic progression , because in the first shape (up left) we have 5+6+1=12 ,1+6+9=16 ,9+6+5=20 so we have sequence:12,16,20 so common difference of successive members is $$d=4$$ for second shape we have:13,16,19 so $$d=3$$ for third shape:18,20,22 ,$$d=2$$ finally for last shape we have $$d=1$$ but this is impossible. I think my answer is wrong because we delete center number in this puzzle and we must use center numbers in all shapes.