(2,-15),(5,y), m=4/5 what does y equal?how do I figure this out? 

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Use the slope and the point you're given to find the equation of the line. y - y1 = m (x - x1) y - (-15) = 4/5 (x - 2) y + 15 = 4/5x - 8/5 Subtract 15 from both sides y = 4/5x - 83/5 Now, plug in 5 for x to get your missing y. y = 4/5(5) - 83/5 y = 4 - 83/5 y = -63/5