First, think about what you did to figure out the first answer when the number of kilowatt hours was 300.
C, the cost of usage, is what you found, when you used 300 as your kilowatt hours.
So, you multiplied 8.77 by 300 and added 7.57
So, C = 8.77 x + 7.57 , but this is only if the kWh or x is not more than 400
Or, the first part of your function is C = 8.77x + 7.57 if x <= 400.
Now, you need to find the part of the piecewise function when x is greater than 400:
So, for this part of the function, think about how you found $5487.77 for 700 kWh.
Think about what you did, and in place of the 700, replace it with x.
Can you figure this part out?