You add x3 and x7 which is x10
Well 6^2 = 36 and (8-4)^2 = 16 so you would do 16 + 7 which is 23 and then 36 - 23 = 13.
Since in y=8, the slope is 0 (because there is no x) and b is 8 in y=mx+b, we know that the perpendicular line is x=(something) because the perpendicular lines of y=<number> are x=<number> due to that y=<number> equations are strictly horizontal while x=<number> equations are strictly vertical. Since we know that x=<number> has to pass through (7, 8), x =7