Directed Graphs and Network Flows · Easy problem

A flow network has arcs $s\to a$ (capacity $5$), $a\to b$ (capacity $2$), $b\to t$ (capacity $6$), all carrying no flow yet. The path $s\to a\to b\to t$ is pushed to its bottleneck. In the resulting residual graph, what is the capacity of the backward arc $b\to a$?