[tex]\displaystyle 2^x+2^{x+1}+2^{x-1}=56 \\ 2^x(1+2^1+2^-^1)=56 \\ 2^x\left(1+2+ \frac{1}{2} \right)=56 \\ 2^x \left( 3+ \frac{1}{2} \right)=56 \\ 2^x \left( \frac{6}{2} + \frac{1}{2} \right)=56 \\ 2^x \cdot \frac{7}{2} =56 \\ 2^x=56: \frac{7}{2} \\ 2^x=56 \cdot \frac{2}{7} \\ 2^x= \frac{112}{7} \\ 2^x=16 \\ 2^x=2^4 \\ x=4[/tex]
