First, lets find power output of the cyclist when there is no wind:

With the wind blowing we find that the speed of wind which the cyclist feels has value V equal to

and makes an angle a with the vector of cyclist'sdisplacement:

Power output with the wind present is thus

Let me explain a little: the force of the wind is proportional to V^2 but work done is . where is the vector of displacement. Power is therefore:

You are sked to compute . If You do the calculations you'll get required answer Sqrt[5].

Good luck