#### Answer

a) $m=\dfrac{1}{3}$
b) If $x$ increases by $3$ units, $y$ will increase by $1$ unit.

#### Work Step by Step

a) The slope of the line that passes throught two points $(x_1, y_1)$ and $(x_2, y_2)$ can be found using the formula:
$m=\dfrac{y_2-y_1}{x_2-x_1}$
Thus, the slope of the line passing through the two given points is:
$m=\dfrac{2-1}{2-(-1)}=\dfrac{1}{3}$
b) Interpreting the slope, we can say that it means how much $y$ changes when $x$ changes. More formally, for every $3$ units change in $x$, $y$ will change by $1$. That is, if $x$ increases by $3$ units, $y$ will increase by $1$ unit.