Suppose there are x ships, then one is x- 1 and the other is x+ 1.

(x- 1)*9 is the class number, and (x= 1)*6 is also the class number.

(x- 1)*9=(x+ 1)*6

9x-9=6x+6

3x= 15

x=5

Then (x-1) * 9 = 36 (x+1) * 6 = 36.

So the class size is 36.