Because one object can have more things in it. I learned it a t school today.
PART A)
Here by force balance along Y direction

Force balance along X direction

now by torque balance



PART B)
now from above equations



now net reaction force of wall is given as


Recall the wave equation,

where c is the speed of the wave (m/s), f is the frequency of the wave (Hz) and λ is the wavelength of the wave (m).

so