Perimeter is the sum of lengths of all outer sides of the shape. Since the rectangle has two pairs of parallel lines of the same length... It is safe to assume that the unmarked side length is 10 ft (just like it's parallel counterpart)
Now it is a simple matter of adding all of the outer sides.
Multiply it just like you are multiplying two numbers with two digits. The at the end of your answer make place the decimal so that there are still numbers in the hundredth place.