Do it systematically.
You know that the “initial KE” of the wind can be expressed in terms of u1, ρ, A1, and Δt.
Do this for “final KE” in terms of u2, ρ, A2, and Δt.
The “change of KE” is “initial KE” minus “final KE”.
Note that A1 u1 = A2 u2 and the “change of KE” can be expressed in a single area.
I believe you can work from there.