Answer:
The acceleration is a = 2.75 [m/s^2]
Explanation:
In order to solve this problem we must use kinematics equations.

where:
Vf = final velocity = 13 [m/s]
Vi = initial velocity = 2 [m/s]
a = acceleration [m/s^2]
t = time = 4 [s]
Now replacing:
13 = 2 + (4*a)
(13 - 2) = 4*a
a = 2.75 [m/s^2]
Answer:
The frequencies that the listener at P will hear a maximum intensity are: 85.75 Hz, 171.5 Hz, 257.25 Hz and 343 Hz.
Explanation:
Path Difference Δx is given as: nλ
where λ = 
Δx can be re-written as: n×
where;
n = integer
v = speed of sound = 343 m/s
f = frequency
Δx = 19.0 m - 15.0 m
Δx = 4.0 m

f = 
f = n × 85.75 Hz
Now; n = 1, 2, 3, 4 ; the frequencies that the listener at P will hear the maximum intensity will be
when n= 1
f = 1 × 85.75 Hz
f = 85.75 Hz
when n= 2
f = 2 × 85.75 Hz
f = 171.5 Hz
when n= 3
f = 3 × 85.75 Hz
f = 257.25 Hz
when n= 4
f = 4 × 85.75 Hz
f = 343 Hz
∴ the frequencies that the listener at P will hear the maximum intensity will be : 85.75 Hz, 171.5 Hz, 257.25 Hz and 343 Hz for n =1,2,3, and 4 respectively.