The following diagram represents the transshipment network of a car supply chain. Numbers on the arcs represent the shipping cost. In this problem, node $2$ is supply center and provides $100$ cars per week. Node $6$ is a warehouse $($transshipment node$).$

Let xij represents how many cars is moved from node i to node j in each week.

Write down the constraint regarding node $7$ for this problem. $[$cons$\mathrm{.}2]$