For the cost function below where C is the cost of producing x units of a product find the marginal$-$cost function. What is the marginal cost at the given value of x$?$ C$($x$)=0\mathrm{.}01$x$3+0\mathrm{.}6$x$2+30$x $+140$; x $=500$ the marginal cost function is C$($x$)$