The total surface area is going to be the area of all sides added together.
In a rectangular prism there are six sides but the good news is they come in pairs meaning you only have to find the area of three sides.
To find the area of a side multiply its side lengths by each other.
After you found the area of three sides multiply each one by 2.
Then take the three numbers you get from that, add em all up, and there’s your answer!!
Hope this helped.