We draw the line y=2 which is the green line below,
and we plot the point P(1,-4)

 

Now we draw the shortest line segment (in black) from from the point
P(1,-4) to the green line:




Now we use the y-axis as a measuring stick and make tick marks
along the black line segment we just drew from the point 
to the line. We make the marks the same distance apart as
the units marked off on the y-axis:



Count the units marked off on the black line segment.
We find it is 6 units long.  So the answer is 6.

Edwin