Answer:
Atomic number (z) = proton number
Atomic number (z) = electron number in a simple element like if the element wasn't in an ionic state
But now the proton number and the electron numbers are different so we can only take that the proton number and the atomic number are equal so,
z = p = 25
mass number = proton + neutron
mass number = 25 + 26
mass number = 51
