Answer:
 
 - age 
 
 --------- 
 
 n (max heart rate) 
 
 .6 *n = target heart rate for exercise
You can use this formula to calculate the target heart rate zone. You haven't provided the maximum heart rate so I am unable to solve your problem.
Step-by-step explanation:
The maximum heart rate for a 14 year old should be 185-206 bpm