[tex] (- 16) {}^{21} = - (16) {}^{21} = - ( 2 {}^{4} ) {}^{21} = \\ \\ = - 2 {}^{4 \times 21} = - {2}^{84} [/tex]
[tex]8 {}^{28} =( 2 {}^{3} ) {}^{28} = {2}^{3 \times 28} = {2}^{84} \\ [/tex]
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[tex]( - 16) {}^{21} - 8 {}^{28} = \\ \\ = - 2 {}^{84} - 2 {}^{84} =- 2 {}^{84} \times 2 = \\ \\ = - 2 {}^{84 + 1} =\boxed{ - {2}^{85}} [/tex]
