The point at which a company's costs equal its revenues is the break-even point, C represents the production cost, in dollars, of x units of a product and $R$ represents the revenue, in dollars, from the sale of $x$ units. Find the number of units that must be produced and sold in order to break even. That is, find
the value of $x$ for which $C=R$
$$\begin{aligned}
&C=3 x+400\\
&R=7 x-600
\end{aligned}$$