Directed Graphs and Network Flows · Medium problem

A flow network has source $s$, sink $t$, and vertices $a,b,c$, with capacities $c(s,a)=7$, $c(s,b)=5$, $c(a,b)=3$, $c(a,c)=5$, $c(b,c)=2$, $c(b,t)=4$, $c(c,t)=7$. A flow of value $9$ is already known to be valid. What is $\operatorname{cap}(S,T) - 9$ for the cut $S=\{s,a,b\}$, $T=\{c,t\}$?