When your are going to find the slope numbers you need to look at what direction the line go. You see it goes down so it must be a negative number. When x is 0 the line is on 5 on the y line. When x is 1 the line is on 1 on the y line. 5 minus 1 is 4. So it must be minus 4.
To find the constant number (the second number) you need to look at where the line started when x is 0. And it started on 5.
