It is a big right triangle whose vertical leg is 1300m, and whose horizontal leg is 12,000m.
Then in that, draw a horizontal line segment that is 350m from the top.
Then, at the intersection of the above line segment with the hypotenuse of the big right triangle, drop a vertical line segment to the horizontal leg of the big right triangle. A smaller right triangle is formed here whose vertical leg is (1300 -350) = 950m and whose horizontal leg is , say, x meters.
This x is what you are looking for.
The big right triangle and the said smaller right triangle are similar, hence proportional.
(vertical leg)/(horizontal leg) of big right triangle = (vertical leg)/(horizontal leg) of smaller right triangle
1300/12,000 = 950/x
1300*x = 12,000*950
x = 12,000*950 /1300 = 8769m ----------answer.