Two things:
1. We don't say 1x or -1x because we are lazy. -1x is just written as -x. It would be like calling you 1 Zainab, though two of you would be 2 Zainabs.
2. Check your answer.
3 = -(7) - 10
3 = -7 - 10
3 = -17
0 = -20
Well, that's not true, so something's not correct with your work.
(-4 - 2)/(2 - 0) = -6/2 = -3
3= -3(7) + b
3 = -21 + b
b = 24
y = -3x + 24
or...
y - 3 = -3(x - 7)
y - 3 = -3x + 21
y = -3x + 24