ContourPlot[((2.64276x10^-6 x-294.264) x 29426.3)/(x^2 y)=5/0.3048,((2.64276x10^-6 x-294.264) x 29426.3)/(x^2 y)=10/0.3048,{x, 0, 80}, {y, 4, 15}]

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