Assuming that the numbers are right next to each other, we have
(x)*(x+1)=156, expanding into x^2+x=156 and then x^2+x-156=0. We can then use the quadratic formula to get x=(-1+-sqrt(1+624))/2=(-1+-25)/2= 12, -13. -13 wouldn't be a page number, so 12 must be one number, and 12+1 (13) is the other!
