Find the b value by plugging in the slope and known coordinates,
-2=0(4)+b
-2= 0+b
-2=b
You can also tell this without calculating since a slope of 0 will always have the same y value. Therefore if (4,-2) is on the line, the y value of the whole line must be -2.
Final answer: y=0x-2, which is y=-2
