The slope is $0.50 since it is that amount per day and the the y-intercept is 15 since when this is graphed, the line intersects the y line at value 15. Hope this helps!
It's simple really
12 can be devided into 3 equal groups of 4
21 a² - 57 a + 30 =
= 3 ( 7 a² - 19 a + 10 ) =
= 3 · 7 · ( a - 5/7 ) · ( a - 2 ) =
= 3 ( 7 a - 5 ) ( a - 2 )