let
x------> the length of the first side
y------> the length of the second side
z------> the length of the third side
we know that
x+y+z=60
x=14 m
y=2x-----> y=2*14-----> y=28 m
z=60-x-y------> z=60-14-28-----> z=18 m
the answer is
the length of the third side is 18 m