I'll provide the first part here:
(i) (a) Split the volume into two parts: 1 refers to left strip and 2 refers to right strip
A1 = 2π(s - t)y
δV1 = 2π(s - t)y δt
but y = f(-t) from the strip
.: δV1 = 2π f(- t)(s - t) δt
A2 = 2π(s + t)y
δV2 = 2π(s + t)y δt
but y = f(t) from the strip
.: δV2 = 2π f(t)(s + t) δt