Explanation:
you know what the slope of the line is ? it is the inclination of the line. how strongly is it going up or down, or is it maybe flat ?
in the formulas the slope is always the factor of x. the slope is excised as ratio of y coordinate change / x coordinate change when going from one point to another.
we see here 2 clear points in my opinion :
(0, -3) and (3, 0)
that is where the line crosses the y and the coordinate axis.
so, going from (0, -3) to (3, 0) :
x changes by +3 (from 0 to 3)
y changes by +3 (from -3 to 0).
so, the slope is
+3/+3 = 1