Lets find the missing sides first
big triangle, we apply the Pythagorean theorem a^2+b^2=c^2
a^=13^2-5^=169-25=144, Square root of a=square root of 144=12
a=12
smaller triangle
5^2-3^2=b^2
25-16=9
b=4
Elm trail is 13km+5km=18km
Dogwood trail=12+5+4+3=24km
correct choices are 2 and 5