y = Mx + b where M<0 and b>0
For x<0, Mx > 0 , which means Mx+b > 0.
So when x<0 y > 0 which lies in quadrant 2.
For x=0, y=b > 0 which is the positive y axis in between quadrants 1 and 2.
for x > 0, Mx < 0.
So Mx + b can be either positive or negative depending on x.
That means the line is in either quadrants 1 or 4.
SUPPOSE the line is in quadrant 3, where x and y are both negative.
Then x<0 and y < 0.
Mx > 0 <--- negative times negative is positive.
since b>0, y = Mx +b > 0.
which contradicts the assumption that y<0.
So the line CANNOT be in quadrant 3.
Therefore the line passes through quadrants 1,2,and 4