15.
[tex]{9}^{ - 8} \times {9}^{10} = {9}^{ (- 8) + 10} = \boxed{{9}^{2} = 81}[/tex]
16.
[tex]( {3}^{5} \times {3}^{ - 3} )^{3} \div {3}^{4} = ( {3}^{5 + ( - 3)} {)}^{3} \div {3}^{4} = ( {3}^{2} {)}^{3} \div {3}^{4} = {3}^{6} \div {3}^{4} = \boxed{ {3}^{2} }[/tex]
