The formula of a line is:

k — stands for tg of line angle
b — stands for the y value of a point where the line crosses y axis.
We can see 2 points:
A (1, -4)
B (3, -5)
We can make a system of equations:

From second equation:

Let's put b in first equation:

Now we can find b:

The answer is:
-0.5x -3.5