The force-deflection relation of a steel helical spring used in an engine is found experimentally as \(F(x)=34.6 x+0.34 x^{2}+0.002 x^{3}\), where the force \((F)\) and deflection \((x)\) are measured in newtons and millimeters, respectively. If the spring undergoes a steady deflection of \(12.7 \mathrm{~mm}\) during the operation of the engine, determine the equivalent linear spring constant of the spring at its steady deflection.