So for something to be perpendicular the gradient must be the negative reciprocal of your original line so your original gradient is 2 so your new gradient will be -1/2. Then you have y = -1/2 x + c . Now you substitute this into the point (4,-1) to get -1 = -1/2 x 4 + c and -1/2 x 4 = -2 so you have -1 = -2 + c so c = 1 so then you put the equation together to get y = -1/2 x + 1
Hope this helps :))
