tolong ya mat .........

No. 17
y + 3 = (1/2) (x + 2)
y + 3 = x/2 + 1
y = x/2 + 1 - 3
y = x/2 - 2
2y = x - 4
2y - x + 4 = 0

no. 18
Eliminasi
-3x + 2y = 8
4x - 2y = -20
------------------ (+)
x = -12

sbtitusi
2x - y = -10
2(-12) - y = -10
y = -24 + 10
y = -14

jadi inilai 
a - 2b
= -12 - 2(-14)
= -12 + 28
= 16

Soal No. 17
[tex]y-y_{1}=m.(x-x_{1}) \\ \\ y-(-3)= \frac{1}{2}.(x-(-2)) \\ \\ y+3= \frac{1}{2}.(x+2) \\ \\y+3= \frac{1}{2}x+1 \\ \\ 2y+6=x+2 \\ 2y-x+6-2=0 \\ 2y-x+4=0 [/tex]
Jawaban (B)

Soal No. 18
x = a
y = b
- 3a + 2b = 8 .......(× 1) ⇒ - 3a + 2b = 8
  2a -    b = - 10....(× 2) ⇒   4a  - 2b = - 20
                                           ---------------------(+)
                                                          a = - 12

2a - b = - 10
2.(-12) - b = - 10
- 24 - b = - 10..........dikali (-1)
24 + b = 10
b = 10 - 24
b = - 14

a - 2b = (-12) - 2(-14)
           = (-12) - (-28)
           = -12 + 28
a - 2b = 16..................Jawaban (A)