a)

$f\left(x\right)=5x-4$  

x	-3	6	-9
y	-19	26	-49

$f\left(-3\right)=5\cdot \left(-3\right)-4=-15-4=-19$  


$f\left(6\right)=5\cdot 6-4=30-4=26$  


$5x-4=-49\mid +4$  

$5x=-45\mid :5$  

$x=-9$  

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b)

$f\left(x\right)=-12x+7$  

x	-4	-7/6	5
y	55	21	-53

$f\left(-4\right)=-12\cdot \left(-4\right)+7=48+7=55$  


$-12x+7=21\mid -7$  

$-12x=14$  

$x=-\frac{14}{12}$  

$x=-\frac{7}{6}$  


$-12x+7=-53\mid -7$  

$-12x=-60\mid :\left(-12\right)$  

$x=5$  

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c)

$f\left(x\right)=\frac{1}{3}x+3\frac{1}{3}$  

x	-6	27	8
y	1 1/3	12 1/3	6

$f\left(-6\right)=\frac{1}{3}\cdot \left(-6\right)+3\frac{1}{3}=-2+3\frac{1}{3}=1\frac{1}{3}$  


$\frac{1}{3}x+3\frac{1}{3}=12\frac{1}{3}\mid -3\frac{1}{3}$  

$\frac{1}{3}x=9\mid \cdot 3$  

$x=27$  


$\frac{1}{3}x+3\frac{1}{3}=6\mid -3\frac{1}{3}$  

$\frac{1}{3}x=2\frac{2}{3}\mid \cdot 3$  

$x=8$