Abook is opened, and the product of the two visible page numbers is found to be 306.

X(x+1) = 306

Distribute the x

x² + x = 306

move all terms to the left and set the equal to zero (be sure to change the sign for all terms moved)

x² + x - 306 = 0
x         -17
x           18

(x - 17)(x + 18)

x =  -18, 17

pg. 17 & 18 is visible

hope this helps