Let's denote three first terms of given arithmetic sequence [tex] a_{1} =18 \\ a_{2}=14 \\ a_{3}=10[/tex] (each next term differs from the previous by -4), then the common difference [tex]d=a_{2}-a_{1}=14-18=-4[/tex]. The formula for the [tex] n^{th} [/tex] term of an arithmetic sequence is [tex] a_{n} =a_{1}+(n-1)d[/tex] and in your case [tex] a_{n} =18+(n-1)(-4)=18-4n+4=22-4n[/tex].
