It's an arithmetic sequence:
[tex]a_1=2\\\\a_2=2-5=-3\\\\a_3=-3-5=-8\\\\a_4=-8-5=-13\\\\a_5=-13-5=-18\\\vdots\\\\\text{The formula of nth term of an arihmetic sequence}\ a_n=a_1+(n-1)d.\\\\\text{We have}\ a_1=2\ \text{and}\ d=-5.\\\\\text{Substitute:}\\\\a_n=2+(n-1)(-5)\qquad\text{use distributive property}\\\\a_n=2+(n)(-5)+(-1)(-5)\\\\a_n=2-5n+5\\\\\boxed{a_n=7-5n}[/tex]
