a skateboarder is coasting down a ramp, and there are three forces acting on her: her W $($magnitude $675-$N$)$ a frictional force f $($magnitude $125$ N$)$ that opposes her motions, and a normal force Fn $($magnitude $612-$N$).$ determine the net work done $($in J$)$ by three forces when she coasts for a distance of $9\mathrm{.}2$m