Answer 1

Wrong syntax. But str+number is invalid in python. str can be concatined to another str only. Try
print(x + str(3))
put string in quotes.

Answer 2

You cannot join string with number only two strings:
x = 123.456
print(x)

x = "This a string"
print(x + str(3))

Answer 3

you can only assign numbers to variables so,
x = This a string
should be in double or single quotes -
x = "This a string"
AND
you can't add string and integer in -
print(x + 3)