Stadium Construction Project Problem: What is the combined slack for activities D and $H?$

$\backslash$table$[[$ACTIVITY$,$IMMEDIATE PREDECESSOR,DURATION $($days$)],[$A$,-,1],[$B$,$A$,4],[$C$,$A$,3],[$D$,$A$,7],[$E$,$B$,6],[$F$,$C$,$D$,2],[$G$,$E$,$F$,7],[$H$,$D$,9],[$I$,$G$,$H$,4]]$

$2$

$1$

$3$

$4$

$0$