Respuesta :

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.

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