giúp mik vs ạ mik đang cần gấp chỉ là bt thoi ạ

$\text{a) Ta có :}$
$\text{x : (-2) = y : 3 = z : 4}$
$\text{=> $\dfrac{x}{-2}$ = $\dfrac{y}{3}$ = $\dfrac{z}{4}$}$
$\text{=> $\dfrac{x.(-3)}{(-2).(-3)}$ = $\dfrac{y.4}{3.4}$ = $\dfrac{z.(-5)}{4.(-5)}$}$
$\text{=> $\dfrac{-3x}{6}$ = $\dfrac{4y}{12}$ = $\dfrac{-5z}{-20}$}$
$\text{Áp dụng tính chất của dãy tỉ số bằng nhau, ta có :}$
$\text{$\dfrac{-3x}{6}$ = $\dfrac{4y}{12}$ = $\dfrac{-5z}{-20}$ = $\dfrac{-3x + 4y - 5z}{6 + 12 - 20}$ = $\dfrac{123}{-2}$}$
$\text{· $\dfrac{x}{-2}$ = $\dfrac{123}{-2}$ => x = 123}$
$\text{· $\dfrac{y}{3}$ = $\dfrac{123}{-2}$ => y = -184,5}$
$\text{· $\dfrac{z}{4}$ = $\dfrac{123}{-2}$ => z = -246}$
$\text{Vậy x = 123 ; y = -184,5 ; z = -246.}$
$\text{Ta có : $2(x - 2)^{2}$ = 0}$
$\text{=> $(x - 2)^{2}$ = 0}$
$\text{=> x - 2 = 0}$
$\text{=> x = 2}$
$\text{Ta có : |2x - y| = 0}$
$\text{=> |2.2 - y| = 0}$
$\text{=> |4 - y| = 0}$
$\text{=> 4 - y = 0}$
$\text{=> 4 = y}$
$\text{A = $-x^{2}$y + $\dfrac{1}{2}$x - 3.$y^{2}$ + 1}$
$\text{A = -$2^{2}$.4 + $\dfrac{1}{2}$.2 - 3.$4^{2}$ + 1}$
$\text{A = -62}$
$\text{Vậy A = -62.}$