Step-by-step explanation:
Atomic Number = Number of protons
Mass Number = Number of protons + Number of neutrons
Isotopes are simply atoms of an element with the same number of protons and different number of neutrons.
First Isotope -- 238U
Number of neutrons = Mass Number - Atomic Number
Number of neutrons = 238 - 92 = 146
Second Isotope -- 235U
Number of neutrons = Mass Number - Atomic Number
Number of neutrons = 235 - 92 = 143