How many neutrons does element x have if its atomic number is 38 and its mass number is 71?

The nucleus of an atom has some protons and some neutrons in it.
The "mass number" is the total number of all particles in the nucleus. 
The "atomic number" is the number of protons in the nucleus.

Element X has 71 particles in the nucleus, and 38 of them are protons.
So (71-38) = 33 of them are neutrons.