Suppose it is 40 degrees inside your refrigerator and 340 degrees inside your oven when the power goes out. the refrigerator temperature starts rising at the rate of 5 degrees per hour. the oven temperature starts dropping at the rate of 45 degrees per hour

I'm assuming that you want to know how many hours until they are the same temperature. Here is the equation you can use to figure it out:

40 + 5h= 340-45h

Here is how I would solve it:

40+ 5h= 340 - 45h
  +45 h          + 45 h

40+50h= 340

-40          -40

50h= 300
 /50   /50

h= 6

It will take 6 hours for the two appliances to reach the same temperature, assuming the power doesn't go out. For reference, they would be at 80 degrees.