Finding the common ratio in an infinite geometric sequence. The first three terms of an infinite geometric sequence are $m - 1$, $6$, $m + 4$, where $m\in\Bbb{Z}$
Write down an expression for the common ratio, $r$.   
Do I divide the second term by the first term to get my answer?  
Edit: I just realized they’re asking for the expression for r. What does that mean and how do I figure that out?

Edit: I just realized you also asked "what does this mean". Frankly, I had to look up the terminology myself:
https://en.wikipedia.org/wiki/Geometric_progression
This informs you that $r(m-1)=6$ and $6r=m+4$. From the second one you get
$$
r=\frac{m}6 + \frac23.
$$
Now putting that into the first gives you
\begin{align*}
&& 6 &= \left(\frac{m}6+\frac23\right)\cdot(m-1) = \frac{m^2}6-\frac{m}6+\frac{2m}3-\frac23 \\
&\Rightarrow& 36 &=m^2+3m-4 \\
&\Rightarrow& 0 &= m^2+3m-40
\end{align*}
That is a quadratic equation in $m$, and the solutions are $m=5$ and $m=-8$.


*

*For $m=5$ you get $r=\frac32$ and the sequence $4,6,9,\ldots$

*For $m=-8$ you get $r=-\frac23$ and the sequence $-9,6,-4,\ldots$.

