Z6 = {0, 1, 2, 3, 4, 5} is not a subring of Z12 since it is not closed under addition
mod 12: 5 + 5 = 10 in Z12 and 10 is not element of Z6.
"Linear Algebra" involves specifically "vector spaces" (abstract mathematical definition, not "Physics" vectors) and "linear transformations" between them. "Abstract algebra" involves "groups", "rings", "fields" and "homomorphisms" and "isomorphisms" between them.
Since this problem is asking specifically about rings, it is a problem in "abstract algebra".