The nucleon number of atom X is 37. It exists as a diatomic molecule, X2. One molecule of X2 contains 34 protons. How many neutrons are present in the nucleus of atom X?

There is one error in question. The total number of proton in X₂ molecule should be 34.
The mass number is also known as nucleon number.
If the mass number of one atom is 37.
Then mass number of X₂ = 2 x 37 =74
In X₂ molecule the total number of proton + neutron = 74.
If total number of proton = 34 
Then total number of neutron = 74-34
                                               = 40
Number of neutron per molecule would be = 40/2
                                                                     = 20