If [tex]d[/tex] is the common difference in an arithmetic sequence, whose [tex]n[/tex]-th term is [tex]a_n[/tex], then
[tex]a_7=a_6+d[/tex]
[tex]a_6=a_5+d[/tex]
[tex]a_5=a_4+d[/tex]
[tex]\implies a_7=a_4+3d[/tex]
We have
[tex]9=18+3d\implies3d=-9\implies d=-3[/tex]
