I don’t get this can someone explain this

Answer:
15.7 units
Step-by-step explanation:
AB = 6, BC = 3
Therefore by Pythagoras theorem
AC = [tex] \sqrt{45}[/tex]
[tex]perimeter \: of \: \triangle \: ABC \\ = 6 + 3 + \sqrt{45} \\ = 9 + 6.70820393 \\ = 9 + 6.7 \\ = 15.7\: units
[/tex]