The third proportional of the following numbers (x - y)², (x² - y²) = ?

$$\eqalign{ & {\text{Let,}} \cr & a = {\left( {x - y} \right)^2}, \cr & b = \left( {{x^2} - {y^2}} \right){\text{and}} \cr & c\,\,{\text{be}}\,{\text{the}}\,{\text{third}}\,{\text{proportional}} \cr & {\text{Therefore}}\,\,\,a:b::b:c \cr & i.e.\,\,\,c = \frac{{{b^2}}}{a} \cr & \Rightarrow c = \frac{{{{\left( {{x^2} - {y^2}} \right)}^2}}}{{\left( {x - y} \right)}} \cr & \,\,\,\,\,\,\,\,\,\,\, = \frac{{{{\left( {x - y} \right)}^2}{{\left( {x + y} \right)}^2}}}{{\left( {x - y} \right)}} \cr & \,\,\,\,\,\,\,\,\,\,\, = \left( {x - y} \right){\left( {x + y} \right)^2} \cr} $$