Since the sequence is geometric, there is some constant \(r\) such that the sequence is recursively given by
\(a_n=ra_{n-1}\)
By this definition, you can recursively substitute into the right hand side the definition for \(a_{n-1},a_{n-2},\ldots\) to find an explicit formula for the \(n\)th term.
\(a_n=ra_{n-1}=r^2a_{n-2}=r^3a_{n-3}=\cdots=r^{n-1}a_1=-625r^{n-1}\)
You know the second term, which means you can find \(r\):
\(a_2=-625r^{2-1}\implies125=-625r\implies r=-\dfrac15\)
So, the 7th term of the sequence is
\(a_7=-625\left(-\dfrac15\right)^{7-1}=-\dfrac1{25}\)
