For the information given in the statement you have
[tex]a_n=a_{n-1}-4[/tex]
Then
[tex]\begin{gathered} a_2=a_1-4 \\ a_2=4-4 \\ a_2=0 \end{gathered}[/tex][tex]\begin{gathered} a_3=a_2-4 \\ a_3=0-4 \\ a_3=-4 \end{gathered}[/tex]
That is, to find the nth term, use the immediately previous term already found
[tex]\begin{gathered} a_4=a_3-4 \\ a_4=-4-4 \\ a_4=-8 \end{gathered}[/tex][tex]\begin{gathered} a_5=a_4-4 \\ a_5=-8-4 \\ a_5=-12 \end{gathered}[/tex]
Therefore, the sixth term of the sequence is
[tex]\begin{gathered} a_6=a_5-4 \\ a_6=-12-4 \\ a_6=-16 \end{gathered}[/tex]
