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\left.\left.\begin{array}{l} f(x)=-2 x+6 \\ f(x)=-2 x-4 \end{array}\right\} \quad \text { parallel } \qquad \begin{array}{l} f(x)=3 x+2 \\ f(x)=2 x+2 \end{array}\right\} \text { not parallel }