High low method fixed stepped cost

first, the stepped up fixed cost of 1000 will b deducted from the total cost of 10000, i.e 10000-1000= 9000
now perform your high and low analysis, (9000-4000)/(1500-500)= 5. this is your variable cost per unit.
remember your formula, y=a+bx. Y=total cost, a=fixed cost n b=variable cost.
Now, finding our A, u can use either of d high or low activity level. Lets use the high here,
9000=a+(5*1500). 9000=a+7500, a=9000-7500, a=1500.
now, A=1500 is your fixed cost before step and (1500+1000)= 2500 is your fixed cost after step up.
now determine your total cost of 800 and 2000.
Use this formulas: TC=1500+5(x) for cost below the step up and TC=2500+5(x) for cost above the step up.
For 800, which is below the stepup, we use TC= 1500+(5*800) which will give you a total cost of 5500; while for 2000 which is above the stepup, we use TC= 2500+(5*2000) which will give you a cost of 12500. That is all, i hope this helps.