$(7 - 2x)(49 + 14x + 4x^2) + 2x(2x - 5)(2x + 5) = 43$
$7^3 - (2x)^3 + 2x(4x^2 - 25) = 43$
$343 - 8x^3 + 8x^3 - 50x = 43$
$-8x^3 + 8x^3 - 50x = 43 - 343$
−50x = −300
x = −300 : −50
x = 6

$100(0,2x + 1)(0,04x^2 - 0,2x + 1) = 5x(0,16x^2 - 4)$
$100((0,2x)^3 + 1) = 0,8x^3 - 20x$
$100(0,008x^3 + 1) = 0,8x^3 - 20x$
$0,8x^3 + 100 = 0,8x^3 - 20x$
$0,8x^3 - 0,8x^3 + 20x = -100$
20x = −100
x = −5