let's detect the sequence, between the numbers there is a difference of 7
then, the next number is always 7 units less
the general formula is
[tex]a_n=(d\times n)+c[/tex]where d is the common diference , n the position of the number,an the result and c a constant
we know d=-7 , now we need c
we can do it replacing an=31 and n=1
because 31 is the result of the fiste number
[tex]\begin{gathered} 31=-7\times1+c \\ 31=-7+c \\ c=38 \end{gathered}[/tex]now the explicit formula is
[tex]a_n=-7n+38_{}[/tex]