`(2x-5) = 15^{2} - 4 . 5^{2}`
$\Rightarrow$ `(2x-5) = 225 - 4 . 25`
$\Rightarrow$ `(2x-5) = 225 - 100`
$\Rightarrow$ `2x-5 = 125`
$\Rightarrow$ `2x = 125 + 5`
$\Rightarrow$ `2x = 130`
$\Rightarrow$ `x = 130 : 2`
$\Rightarrow$ `x = 65`
$\text{Vậy}$ `x = 65`
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`b, 3x + 2 - 3x . 2^{3} = 27`
$\Rightarrow$ ` 3x + 2 - 3x . 8 = 27`
$\Rightarrow$ ` 3x + 2 - 24x = 27`
$\Rightarrow$ ` 3x - 24x = 27 - 2`
$\Rightarrow$ ` -21x = 25`
$\Rightarrow$ ` x = 25 : (-21)`
$\Rightarrow$ ` x = 25/(-21)`
$\text{Vậy}$ ` x = 25/(-21)`
