The y-intercept is 8 because (0,8) and if you do rise/run for the slope you get 2/3. 0-(-4)/(12-6)=4/6=2/3
Edit:
I just graphed the points you had and then used the slope to go backwards. So I did (-4,6) and went down 2 and left 3 (from the 2/3 slope rise/run), that gives you (-6, 3) and then I did it again I went down 2 and left 3 which gives you (-8,0). The y intercept is where x=0 so then it would be y=-8 as the y-intercept. To find the slope you just subtract the y-values (6-0) for the last 2 and put that answer (6) over the run which you get from subtracting the x values (21-12) again for the last two. That gives you 9 so it is 6/9. If you simply that you get 2/3 because 6/3=2 and 9/3=3. So you divide the top and bottom of 6/9 by 3 to get 2/3 for the slope. I gave the equation in slope-intercept form which is y=mx+b and m=slope, b=y-intercept. Let me know if it still doesn't make sense i don't know how to say it and it won't let me comment
