Times 4 because, 5 times 4 is 20, 20 times 4 is 80 and so on.
Simply distribute the -3 across the b and -7.
-3(b - 7) = (-3 * b) + (-3 * -7) = -3b + 21
Hope this was helpful, if it wasn't just drop me a DM explaining what didn't make sense.
Answer:
m = -12 / 6 = -2 / 1 = -2
So its -2
Y = -3x + b
Plug in points
4 = -3(-2) + b
4 = 6 + b, b = -2
Solution: y = -3x - 2