An electric clothes dryer is rated at 2,800 W. How much energy does it use in 35 min?

You can use the following formula to solve this problem: Electric Energy = Power x Time.  
You have already been given the power. It's 2,800 watts. 1 watt of power is equivalent to 1 Joule of energy per second.
That means you need to convert 35 minutes into seconds. Multiply 35 minutes x 60 seconds/ minute and you get 2,100 seconds.  
Now multiply that answer by the 2,800 watts of power being used.  
2,800 x 2,100 = 5,880,000 Joules
The electric dryer in question is using 5,880,000 Joules of energy during that 35 minute time period. Compared to anything else in the house, that clothes dryer is probably the the highest electric energy consumer on the property. I've attached a link below that explains a few other formulas regarding electricity consumption, usage, and how the power company uses kilo-watt hours instead of joules.