[latex] \frac{1}{2^a} + \frac{1}{2^{a-1}}+ \frac{1}{2^{a-2}}=28[/latex]

[latex] \frac{1}{2^{a}}+\frac{1}{2^{a-1}}+\frac{1}{2^{a-2}} =28\\\\ \frac{1}{2^{a}}+\frac{1}{2^{a}\cdot 2^{-1}}+\frac{1}{2^{a}\cdot 2^{-2}} =28\\\\ \frac{1}{2^{a}}+\frac{2}{2^{a}}+\frac{4}{2^{a}}=28 \\\\ \frac{7}{2^{a}} =28\\\\2^{a}=\frac{7}{28}\\\\2^{a}=\frac{1}{4}\\\\2^{a}=2^{-2}\\\\a=-2[/latex]