Answer: Approximately 9.526279 meters
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Step-by-step explanation:
See the diagram below.
Draw a horizontal line to mark points A, B and C on it
A = dartboard
B = Jack's location
C = midpoint of A and B
Segment AB is the 11 meter horizontal distance from Jack to the dartboard. Half of this is BC = 11/2 = 5.5 meters.
Then just above point C, we'll have a fourth point D. Segment CD is the unknown max height of the dart. Let's call this x.
The dart will begin at point B, go upward in a parabolic arc to get to point D (the peak of the mountain), then come back down to arrive at point A.
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Focus on triangle BCD. Angle B = 60 degrees is the angle of elevation that Jack uses to throw the dart.
We'll use the tangent ratio to connect the opposite side x to the adjacent side 5.5

Make sure your calculator is in degree mode.
Segment CD is approximately 9.526279 meters which is the max height the dart reaches.
See the diagram below.