Diketahui f(x)=ax+b.Jika f(4)=-13 dan f(-7)=42,maka nilai f(9) adalah...

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f(x) = ax+b
f(4) = a(4)+b
-13 = 4a+b.....1
f(-7) = a(-7)+b
42 = -7a+b .....2

eliminasi persaman 1&2
-13 = 4a+b
42 =-7a+b  - 
-55 = 11a
-55/11 = a
-5 = a

subtitusi a = -5
-13 = 4a+b
-13 = 4(-5)+b
-13 = -20+b
20-13 = b
7 = b

f(x) = ax+b
f(9) = -5(9)+7
= -45+7
= -38