Answer:
10.5
Step-by-step explanation:
If the perimeter is 34, all the sides add to 34
12 + 5 + 7 + 4+ DE = 34
28 + DE = 34 Subtract 28 from both sides.
28 - 28 + DE = 34 - 28
DE = 6
Set up the the proportion
12 / 21 = 6/JI Cross multiply
12*JI = 21 * 6 Combine the right
12*JI = 126 Divide both sides by 12
JI = 126/12 Divide
JI = 10.5
