The formula for perimeter is simple: just add all the sides.
If the three sides of the triangle-shaped flower bed are lengths 3 1/2 yards, 2 1/2 yards and 4 1/2 yards, then we just add these together to get the perimeter.
3.5 + 4.5 + 2.5 = 10.5
Therefore, the perimeter of the flower bed is 10 1/2 yards.
Hope this helped:)
~Ash
P.S. Would you like me to also answer the second question on the page?