equation solving - Solve sum with indices

I'm trying to check some equations I did on paper, but I have troubles getting Mathematica to solve them.

The equation (with Latex):

My input: Sum[2 Subscript[x, i] (b Subscript[x, i] - Subscript[y, i]), {i, 1, n}] == 0

If I want to solve for b with Solve[%, b] it says:

Solve::nsmet: This system cannot be solved with the methods available to Solve.

Are the indices the problem? I tried it with x[i] and y[i], but that didn't work either.

What am I missing?

Comments

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