$(x - 17)(x + 17) = x^2 + 6x - 49$
$x^2 - 289 = x^2 + 6x - 49$
$x^2 - x^2 - 6x = 289 - 49$
−6x = 240
x = 240 : (−6)
x = −40

(1,2x − 4)(1,2x + 4) − (1,3x − 2)(1,3x + 2) = 0,5x(8 − 0,5x)
$1,44x^2 - 16 - (1,69x^2 - 4) = 4x - 0,25x^2$
$1,44x^2 - 16 - 1,69x^2 + 4 = 4x - 0,25x^2$
$1,44x^2 - 1,69x^2 + 0,25x^2 - 4x = 16 - 4$
$1,44x^2 - 1,69x^2 + 0,25x^2 - 4x = 16 - 4$
−4x = 12
−4x = 12 : (−4)
x = −3