The incline of a roller coaster is 2 times as long as its elevation, and the horizontal length of the roller coaster is 11 m more than the elevation. what is the length of the incline in meters?

Let 
x-------> the horizontal length of the roller coaster
y------> the elevation of the roller coaster
c------> the incline of a roller coaster

we know that
applying the Pythagoras theorem
c²=x²+y²-----> equation 1
c=2y-----> equation 2
x=11+y----> equation 3

substitute equation 2 and equation 3 in equation 1
[2y]²=(11+y)²+y²
4y²=11²+22y+y²+y²
2y²-22y-121=0

using a graph tool-----> to resolve the second order equation
see the attached figure

the solution is 
y=15.026 m

c=2*y-----> c=2*15.026----> c=30.052 m

the answer is
30.05 m