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As Alan said, plug in "z+3" instead of "x" at every occurrence of "x".
In the given case, it works this way:
f(z+3) = 2*(z+3)^2 + 4*(z+3) - 1.
Next, you must simplify it.
It is simple job, and I leave it to you to complete it.
In order for to get full understanding, you should complete it on your own at least once.
If you want, you can post your answer to the forum - somebody of the tutors will verify it . . .
If you do, then refer correctly to THIS post.
For it, you may use the ID number of the post, which (the ID number) is 1144236.