The equation of the straight line is y=mx+q where q is the number on the y-axis where the line passes, as you can see it is -3. It turns into:
y=mx+(-3) -> y=mx-3
Then consider a point on the line and take the coordinates, such as the point with coordinates (-2;-4), so now you know that:
x=-2 and y=-4
At this point you put these values into the equation:
y=mx-3
-4=m(-2)-3
then solve:
-4=-2m-3
-2m=+3-4
-2m=-1
m=+1/2
Put the value of m into the equation and you found it:
y=1/2x-3
