Hey there mate :)
Please check the attached image for answer.
Assuming the coin is not weighted and is a fair and standard coin - the chance of flipping head is 1/2. You can either flip head or tails, there are no other possible outcomes.
HK consists of two part, HJ and JK
HK = HJ + JK
We know that:
HK = 12.2ft
JK = 3.1ft
HJ = ?
Let's plug our values into the equation above.
12.2ft = HJ + 3.1ft
Subtract 3.1ft from both sides.
9.1ft = HJ