If there are three sides and each side is 13 meters the do 13 times 3. Which equals 39 . Your answer is 39 m.
Assuming you are trying to get price per pound.
Divide 750 by 5 which equal 150 or $1.50
Each term is being divided by -3, so the next three terms would be:
-1/3, 1/9, -1/27