[tex]\displaystyle\bf\\\frac{y^{2} -18y+81}{y^{2} -81}:\frac{5y-45}{y^{2}+9 y} =\frac{(y-9)^{2} }{(y-9)(y+9)} :\frac{5\cdot(y-9)}{y\cdot (y+9)} = \\\\\\=\frac{(y-9)^{2} }{(y-9)(y+9)} \cdot\frac{y\cdot(y+9)}{5\cdot (y-9)} = \frac{y}{5} \\\\\\y=54\\\\\\\frac{y}{5} =\frac{54}{5} =10,8\\\\\\Otvet \ : \ 10,8[/tex]