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The easiest formula to use here is y=mx+b 

You can plug in -8 for b, making it y=mx-8

Next you must find the slope, which is m. The formula for slope is y2-y1/x2-x1 (the numbers are subscripts) 

Use the point (4,2) in the slope formula, and since you only have one point you use zero for y1 and x1 
This would look like 4-0/2-0 which = 4/2= 2 

So your slope is 2. M=2. 

Because you are finding the perpendicular of a line you use the reciprocal of your slop, which = -2. 

Now you have y=-2x-8 which is your answer. 

Hope this helped:)