You can put this solution on YOUR website! If g(x)=3x+8, solve g[g(x)]=g(x) for x.
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g[3x+8] = 3x+8
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3(3x+8)+8 = 3x+8
9x+24 + 8 = 3x+8
6x + 32 = 8
6x = - 24
x = -4
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Cheers,
Stan H.
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