Concept:The slope formula relates two points: m=x2−x1y2−y1.Explanation:Take P(2,−8) as (x1,y1) and Q(1,y) as (x2,y2).So m=1−2y−(−8).Given m=−4, so −4=−1y+8.Multiplying both sides by −1 gives 4=y+8.Therefore, y=4−8=−4.Answer:y=−4, which is option C.