The slopes of perpendicular lines are called negative reciprocals of each other. You may have heard of this as negative opposites. Basically, if you have one line with a slope of 3/4, then a line perpendicular to it would have a slope of -4/3.
The original equation they give you is in point-slope form: y-y1=m(x-x1) with 1/2 being the slope of the line.
If the slope of the original line is 1/2, then the slope of the line perpendicular to that would be -2/1 or just -2.
The problem also gives you the y-int. (0,4).
Now you have all the information needed to put it in slope intercept form.
slope intercept form is y=mx+b where m is slope and b is the value of the y intercept.
From previous calculations we know that the slope of our line is -2 and the y-intercept is 4.
Substitute in those values: y=-2x+4
that's you answer:)
