We will assume the tunnel is a cylinder with radius r = 5 m and length l.
The formula for surface area of the playground tunnel is:
S = 2πrl
S = 2π*5*l
S = 10πl
And we also know that the area of the tunnel is S = 54 m^2 thus we have:
10πl = 54
π = 3.14
10*3.14*l = 54