Final answer:
The parking strip must be lengthened by 173 feet, which is found by subtracting the current length of 145 feet from the desired length of 318 feet.
Step-by-step explanation:
The question involves a simple subtraction problem to determine how much longer a parking strip needs to be to achieve a certain length. If the current length of the parking strip is 145 feet and it needs to be 318 feet long, the length by which it must be extended is the difference between these two lengths.
To find the difference, subtract the original length from the desired length:
318 feet (desired length) - 145 feet (current length) = 173 feet.
Therefore, the parking strip must be lengthened by 173 feet.