`a.`

$7^{2x-6} = 49$

$7^{2x-6} = 7^2$

`2x -6 = 2`

`2x = 2 + 6`

`2x = 8`

`x=4`

Vậy `x=4`

`b.`

$5 + 3^{x+1} = 86$

$3^{x+1} = 86 - 5$

$3^{x+1} = 81$

$3^{x+1} = 3^4$

`x + 1 = 4`

`x = 3`

Vậy `x=3`

`c.`

$2^{x+1} - 2^x = 32$

$2^x \cdot 2 - 2^x = 32$

$2^x \cdot (2 - 1) = 32$

$2^x \cdot 1 = 32$

$2^x = 32$

`2^x = 2^5`

Vậy `x = 5`

`d.`

$5^x + 5^{x+2} = 151$ 

$5^x + 5^x \cdot 25 = 151$

$5^x \cdot (1 + 25) = 151$

$5^x \cdot 26 = 151$

$5^x = \dfrac{151}{26}$

Vậy ko có giá trị `x` thỏa mãn