The line through (a,b) and (c,d) is
(c-a)(y-b) = (d-b)(x-a)
Picking the last two points,
(-2 - -6)(y - -21) = (-14 - -21)(x - -6)
4(y+21) = 7(x+6)
When x=0
4y + 84 = 42
4y = -42
y = -21/2
Answer: -21/2
We never used the first point, let's check it's on the line
4(y+21) = 7(x+2)
4(-28 + 21) = -28
7(-10 + 6) = -28, good
