Answer :
Suppose 3x + 7 is even when x is even
then 3x + 7 = 2m where m is some integer
3x = 2m - 7
which means that 3x is odd because 2m - 7 must be odd
but x is even and 3x is even so our original assumption is wrong
So 3x - 7 must be odd when x is even.
then 3x + 7 = 2m where m is some integer
3x = 2m - 7
which means that 3x is odd because 2m - 7 must be odd
but x is even and 3x is even so our original assumption is wrong
So 3x - 7 must be odd when x is even.