tolong bantu jawab yah

No. 2

x + y - z = 1 ............(1)
2x + y + z = 11.........(2)
x + 2y + z = 12.........(3)

dari (1) dan (2)
x + y - z = 1
2x + y + z = 11
---------------------- (+)
3x + 2y = 12.................(4)

dari (1) dan (3)
x + y - z = 1
x + 2y + z = 12
--------------------- (+)
2x + 3y = 13.................(5)

dari (4) dan (5)
9x + 6y = 36
4x + 6y = 26
----------------- (-)
5x = 10
x = 2

subtitusi
2x + 3y = 13
2(2) + 3y = 13
3y = 13 - 4
3y = 9
y = 3

subtitusi
x + y - z = 1
2 + 3 - z = 1
z = 5 - 1
z = 4

HP = {(2,3,4)}

No. 3
2x + y + z = 4700 ....................(1)
x + 2y + z = 4300 ....................(2)
3x + 2y + z = 7100 ..................(3)

dari (1) dan (2)
2x + y + z = 4700
x + 2y + z = 4300
------------------------- (-)
x - y = 400 ....................(4)

dari (1) dan (3)
2x + y + z = 4700
3x + 2y + z = 7100
--------------------------- (-)
x + y = 2400 ..................(5)

dari (4) dan (5)
x - y = 400
x + y = 2400
------------------ (+)
2x = 2800
x = 1400

subtitusi
x - y = 400
1400 - y = 400
y = 1000

subtitusi
2x + y + z = 4700
2(1400) + 1000 + z = 4700
z = 4700 - 3800
z = 900

jadi harga
buku tulis = 1400
ppensil = 1000
penghapus = 900

No. 4
misalkan
m = 1/x
n = 1/y
r = 1/z
maka
m + n = 2 ...........(1)
n - r = -3 ............(2)
m - r = 2 .............(3)

dari (1) dan (2)
m + n = 2
n - r = -3
------------ (-)
m + r = 5 ..........(4)

dari (3) dan (4)
m - r = 2
m + r = 5
--------------- (+)
2m = 7
m = 7/2

m - r = 2
m + r = 5
--------------- (-)
2r = 3
r = 3/2

subtitusi
m + n = 2
7/2 + n = 4/2
n = -3/2

jadi
m = 1/x
7/2 = 1/x
x = 2/7

n = -3/2
1/y = -3/2
y = -2/3

r = 3/2
1/z = 3/2
z = 2/3

jadi 
x + y + z
= (2/7) + (-2/3) + (2/3)
= 2/7

No. 6
x = 2
y = 1 , maka
2a - b = 6 .............(1)
4a + 3b = 2 ...........(2)

eliminasi
4a - 2b = 12
4a + 3b = 2
----------------- (-)
-5b = 10
b = -2

subtitusi
2a - b = 6
2a + 2 = 6
2a = 4
a = 2

a² + b²
= (2)² + (2)²
= 4 + 4
= 8