In Javascript, you can accept an input value by using the prompt() function and saving the input into a variable. In the following lines of code, I have declared the three variables at the beginning and then prompted the user to enter a value for each and saved the values in the correct variables. In Javascript length is a keyword so I used len instead.
He runs up the stairs, so he needs a total amount of energy = mgh = 5880 Joules.
For the rate: since he goes from zero energy to 5880 Joules over 8 seconds...
Joules/sec = 5,880/8 = 735 W