[tex] {7}^{x + 2} - 3 \times {7}^{x + 1} - 20 \times 7= 392[/tex]
[tex] {7}^{x} \times {7}^{2} - 3 \times {7}^{x} \times 7 - 20 \times 7= 392 [/tex]
[tex]49 \times {7}^{x} - 21 \times {7}^{x} - 20 \times 7 = 392 \: | \div 7[/tex]
[tex]7 \times {7}^{x} - 3 \times {7}^{x} - 20 = 56[/tex]
[tex]7 \times {7}^{x} - 3 \times {7}^{x} = 56 + 20[/tex]
[tex]7 \times {7}^{x} - 3 \times {7}^{x} = 76[/tex]
[tex] {7}^{x} (7 - 3) = 76[/tex]
[tex] {7}^{x} \times 4 = 76 \: | \div 4[/tex]
[tex] {7}^{x} = 19[/tex]
[tex]ln {7}^{x} = ln19[/tex]
[tex]xln7 = ln19[/tex]
[tex]x = \frac{ln19}{ln7} [/tex]
