Answer:
f(x) = x² + 2x + 1
Explanation:
you know how to multiply 2 expressions ?
let's say in general we have
(a + b)(c + d)
you take one part of one expression and multiply it with all parts of the other expression, then you take the second part of the first expression and multiply it with all parts of the other expression, then a potential third part, then a fourth part and so on, and you add all these things together (well, depending on the actual signs, of course).
so, we get for this simple generic example
a×c + b×c + a×d + b×d
now we use that understanding for our question here.
(x+1)(x+1) = x×x + 1×x + x×1 + 1×1 = x² + x + x + 1 = x² + 2x + 1