It is returning the value, you simply are not capturing it or doing anything with it. For instance:
def triangle_area(b, h):
return 0.5 * b * h
output = triangle_area(20, 10) # capture return value
print(output)
Or as others have suggested, just pass the result straight to the print function if that's what you're looking to do:
def triangle_area(b, h):
return 0.5 * b * h
print(triangle_area(20, 10)) # capture return value and immediately pass to print function
Note that the output will be lost if passed straight to print(). If you want to use it later in your program, the first code block is more appropriate
