Answer: -9
Step-by-step explanation: To find the value of u, we can use the slope formula to calculate the slope of the line that goes through the points (9, u) and (10, -1). The slope formula is: slope = (y2 - y1)/(x2 - x1). In this case, we have: slope = (-1 - u)/(10 - 9) = (-1 - u)/1. Since the slope of the line is given as 8, we can set this equal to the value we calculated above and solve for u. This gives us: 8 = (-1 - u)/1. Multiplying both sides by 1 gives us: 8 = -1 - u. Adding 1 to both sides gives us: 9 = -u. Finally, negating both sides gives us: u = -9. Therefore, the value of u is -9.