Answer
$-\dfrac {5}{2}$
Work Step by Step
$\left( -7\right) -\left( -\dfrac {9}{2}\right) =\left( -\dfrac {9}{2}\right) -\left( -2\right) =\left( -2\right) -\dfrac {1}{2}=\dfrac {1}{2}-3=-\dfrac {5}{2}$
So this is arithmetic sequence
$d=a_{n+1}-a_{n}=\left( -7\right) -\left( -\dfrac {9}{2}\right) =-\dfrac {5}{2}$
