Answer is: the identity of element Y is B (boron).
A(Y) = 11; mass number.
A = p⁺ + n°; mass number is the total number of protons and neutrons in a nucleus.
n°(Y) = 6; number of neutrons in atom Y.
p⁺(Y) = A(Y) - n°(Y).
p⁺(Y) = 11 - 6.
p⁺(Y) = 5; number of protons in atom Y.
Z = p⁺; atomic number.
Atomic number is the number of protons, which is characteristic of a chemical element, boron is an element with atomic number 5.