Use the equation s = s0 + vt, which comes from physics. This is a case where the acceleration is zero.
new position = old position + velocity times time
5.40 m = 19.9 m + (-0.418 m/s)t
Subtr. 19.9 from both sides: -14.5 m = (-.418 m/s)* t
Divide both sides to -.418 to isolate t: t = 34.69 seconds (answer)