Answer:
[tex] \: \: \: \: \: \: 15 - d = 6 \\ = > - d = 6 - 15 \\ = > - d = - 9 \\ = > d = 9[/tex]
Verification :
[tex] \: \: \: \: \: \: 15 - d = 6 \\ = > 15 - 9 = 6 \\ = > 6 = 6[/tex]
