Step up cost or change in variable

A business has experienced the following labour costs :

Code:
Output Cost
(units) ($)
7000 86000
12000 141000
9000 97500

Fixed costs increase by $15000 for output in excess of 10000 units.
Using the high low method what is the estimated cost of producing 14000 units?
A. 142000
B. 157000
C. 163000
D. 178000

Hello sir I believe the answer is B 157000 which we got by taking the highest unit and lowest unit 12000 and 7000units
but as per my understanding if there is a change in variable or fixed cost you take the high low units where the units are not affected by the change. As in this case should be 9000 and 7000 and then adjust for the change.
Could you please explain why we did not do so in this

………………………….

like in this we took the units which were not affected by the change which is 6000 and 7500

Q.An organisation has the following total costs at three activity levels:

Output Cost
(units) ($)
4000 40800
6000 50000
7500 54800

Variable cost per unit is constant within this activity range and there is a step up of 10% in the total fixed costs when the activity level exceeds 5,500 units.

What is the total cost at an activity level of 5,000 units?

Calculate the variable cost per unit by comparing two output levels where fixed costs will be the same:

Variable cost per unit = [(54,800 – 50,000) / (7,500 – 6,000)] = $3.20

Total fixed cost above 5,500 units = [54,800 – (7,500 x 3.20)] = $30,800

Total fixed cost below 5,500 units = 30,800/110 x 100 = $28,000

Total cost for 5,000 units = [(5,000 x 3.20) + 28,000] = $44,000