a, (-12)-(13-x)=-15-(-17) b, x-43=(35-x)-48 c, -(x-6+85)=(x+51)-54 d, 305-x+14=48+(x-23) e, -(35-x)-(37-x)=33-x

Bạn tham khảo: 

a, $(-12)-(13-x)=-15-(-17)$ 
$⇔ -12 -13 +x = -15 + 17$
$⇔ x = -15 + 17 + 12 + 13$
$⇔ x = 27$
 
b,$ x-43=(35-x)-48$ 
$⇔ x – 43 = 35 – x – 48$
$⇔ x + x = 35 – 48 + 43$
$⇔ 2x = 30$
$⇔ x = 15$

c, $-(x-6+85)=(x+51)-54$ 
⇔ $-x + 6 – 85 = x + 51 – 54$
⇔ $-x – x = 51 – 54 – 6 + 85 $
⇔ $-2x = 76$
⇔ $x = -38$

d,$ 305-x+14=48+(x-23)$ 
$⇔ 305 – x + 14 = 48 + x – 23$
$⇔ -x – x = 48 – 23 – 305 – 14$
$⇔ -2x = -294$
$⇔ x = 147$

e, $-(35-x)-(37-x)=33-x$
$⇔ -35 + x – 37 + x = 33 – x$
$⇔ x + x + x = 33 + 35 + 37$$
$⇔ 3x = 105$
$⇔ x = 35$