The prefix "iso" indicates "the same as", so isoneutronic means the same number of neutrons.
The mass number tells you the number of nuclides that comprise the nucleus of an atom. These are protons and neutrons. The atomic number tells you the number of protons on the nucleus, so:
mass number = number of protons + number of neutrons.
A: 76 = 32 + number of neutrons, so number of neutrons = 76 - 32 → 44
B: Has same number of neutrons as A, so 77 = number of protons + 44
Hence number of protons = 77 - 44 → 33. This is the atomic number of B.