Split into the two statements:

(1)If is a multiple of 7, then is a multiple of 7 as well.

Since is a multiple of 7, then for some . Now let's plug this in:

Obviously, this is a multiple of 7 so we're done.

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(2)If is a multiple of 7, then is a multiple of 7 as well.

Since is a multiple of 7, then for some

Plug this into and the procedure is the same as above.