electricity costs 7.57 a month plus 8.77 per kilo watt hour for the first 400 kWhr and 6.574 per kWhr for all useage over 400 in a month.

what is the cost for 300 kWhr? 2638 What is the cost of 700? 7.57+8.77 times 400 and 6.574 times 300? 5487.77 I think. I need to write a piecewise function x= kWhr used per month and c=cost of useage. please help me I do not understand