It's an arithmetic sequence:
[tex]a_1=-1\\a_2=a_1-3\to a_2=-1-3=-4\\a_3=a_2-3\to a_3=-4-3=-7\\a_4=a_3-3\to a_4=-7-3=-10\\\\a_1=-1,\ d=-3[/tex]
The general term of an arithmetic sequence is equal:
[tex]a_n=a_1+(n-1)d[/tex]
Substitute:
[tex]a_n=-1+(n-1)(-3)[/tex] use distributive property
[tex]a_n=-1+(n)(-3)+(-1)(-3)\\\\a_n=-1-3n+3\\\\a_n=2-3n[/tex]
Answer: [tex]\boxed{a_n=2-3n}[/tex]
