It is implied that the king had Joab brought before him so that he could speak to him. The full meaning of this can be made clear. AT: "Then the king summoned Joab and said to him" (See: [[rc://en/ta/man/translate/figs-explicit]])
This phrase is used to draw someone's attention to what is said next. AT: "Listen"
# this thing
This refers to what Joab wants the king to do. The full meaning of this statement can be made clear. AT: "what you want me to do" (See: [[rc://en/ta/man/translate/figs-explicit]])