Simplify rational expressions (advanced)

Factor the numerator and the denominator of a rational expression using advanced methods, and cancel out common terms.

Problem

Simplify the following rational expression and express in expanded form.
start fraction, k, start superscript, 2, end superscript, minus, y, start superscript, 2, end superscript, divided by, 3, k, plus, 3, y, end fraction, equals