y must equal something,So solve from Y",This is confusing..

The slope-intercept form of a line:
[tex]y=mx+b[/tex]
m - slope
b - y-intercept → (0, b)
The formula of a slope:
[tex]m=\dfrac{y_2-y_1}{x_2-x_1}[/tex]
We have two points from the graph (-4, -1) and (0, 3) → b = 3.
Substitute:
[tex]m=\dfrac{3-(-1)}{0-(-4)}=\dfrac{4}{4}=1[/tex]