you have to use the pythagorean (definatly spelled wrong) therom to solve for the side of the "triangle" formed.
\(\displaystyle a^2 + b^2 = c^2\)
c is the longest side of this right triangle, and is called the hypotnuse (also spelled wrong). You have a & b, just solve for c and add it to the top part. Dont forget to double c since the otherside if the ride is the same legnth.
You also need to recognize that this is a 45-45-90 triangle, therefore you know that a&b are both 15 meters.