Answer
The missing number is $-4$
$m=-1$
Work Step by Step
If the value of x increases by 1 unit (from 2 to 3), the value of y decreases by 1 units.
Therefore the value of $y$ when $x$ is $5$ is $-2-2=-2-2=-4$ (Because x is increased by 2 units)
m is the change in $y$ divided by the change in $x$.
Therefore $m=-1/1=-1$
