Answer
$ a_{n}=0.0004(-10)^{n-1}$ and $ a_{7}=400$
Work Step by Step
The general formula to find the nth term of a Geometric sequence is given as: $ a_{n}=a_1r^{n-1}$
$ r=\dfrac{a_2}{a_1}=\dfrac{-0.004}{0.0004}=-10$ and $ n=7$
Now, $ a_{n}=0.0004(-10)^{n-1}$
Now, $ a_{7}=0.0004(-10)^{7-1}=400$
Our answers are: $ a_{n}=0.0004(-10)^{n-1}$ and $ a_{7}=400$
