1) $$y^9 \cdot y^6 = y^{9+6} = y^{15}$$
2) $$y^9 : y^6 = y^{9-6} = y^3$$
3) $$(y^6)^6 = y^{6\cdot6} = y^{36}$$
4) $$\frac{y^{19} \cdot (y^5)^2}{y^{26}} = \frac{y^{19} \cdot y^{5\cdot2}}{y^{26}} = \frac{y^{19} \cdot y^{10}}{y^{26}} = \frac{y^{19+10}}{y^{26}} = \frac{y^{29}}{y^{26}} = y^{29-26} = y^3$$
Ответ: 1) $$y^{15}$$; 2) $$y^3$$; 3) $$y^{36}$$; 4) $$y^3$$