Remember no mass can be lost or gained throughout the reaction. With this in mind we can find out the mass of the water on the right side of the equation:
4.92 - 2.40 = 2.52.
The molar mass of water (H2O) is 18 (1+1+16) so to find the number of moles of it you do the molar mass divided by the actual mass:
18.00 / 2.52 = 7.14
The value will never be perfect because of experimental error, so round it to 7. So there are 7 moles of water on the right side of the equation, so n = 7.
Hope that helps