Both regression lines pass through the means of x and y. Solving for the intersection, which gives the mean, you get

mean of x = 13
mean of y = 6

Hence,

8th x value = 8 x 13 - all other x values = 10
8th y value = 8 x 6 - all other y values = 8

|#|x|y|Y|(y - Y)²|
|1|10|9|8.1|0.81|
|2|11|8|7.4|0.36|
|3|12|7|6.7|0.09|
|4|11|6|7.4|1.96|
|5|17|5|3.2|3.24|
|6|14|4|5.3|1.69|
|7|19|1|1.8|0.64|
|8|10|8|8.1|0.01|

Sum of (y - Y)² = 8.8

The value would be greater than or equal to 8.8 as a = -0.7 and b = 15.1 is solved to give the lowest possible value of the sum square of deviations Σ(y - Y)² can take.