Problem 1
A python function returns a value when called.
Asuka_wataki's program only defines a function, it doesn't call it.

Problem 2
The python function arguments, in this case vp, vs and t, give the value when called.

Problem 3
When executing a program contained in a file, it will not be output unless a print statement is used.

Problem 4
This program does not require import math.

If you modify these, it will be as follows.

def ex3_4 (vp, vs, t):
    p = (vp * vs)/(vp-vs) * t
    return (p)
print (ex3_4 (6, 3, 2))

When executed in interactive mode, it will be as follows.

$python
Python 3.8.3 (default, Jul 2 2020, 17:30:36) [MSC v.1916 64 bit (AMD64)] :: Anaconda, Inc. on win32
Type "help", "copyright", "credits" or "license" for more information.
>>>def ex3_4 (vp, vs, t):
... p = (vp * vs)/(vp-vs) * t
... return (p)
...
>>>print (ex3_4 (6, 3, 2))
12.0

When executed in command execution mode, it will be as follows.

$python ex3_4.py
12.0

It has been calculated.
def ex3_4 (vp, vs, t) is
We just define a function called ex3_4 with vp, vs, t as variables, so we need to call that function.

Please check the following additions.

import math
def ex3_4 (vp, vs, t):
# vp = 6
# vs = 3
# t = 2
    p = (vp * vs)/(vp-vs) * t
    return (p)
ans = ex3_4 (6,3,2) #function call
print (ans) #Display result

By the way, vp, vs, t can be operated from the outside, so I commented them out.
If you give a number in ex3_4 (,,), it will return the number.