mathematics - Equation "solved through" or "solved by"?

Of the two, I personally think "solved by" sounds better.

The comments on your question provide some excellent alternatives as well. "has the solutions/roots" and "is satisfied by" are great, though I would prefer the former if you're supplying the full list of possible solutions as the latter could be ambiguous. E.g. "x²=4 is satisfied by x=2" is completely accurate, although it doesn't provide all possible roots to the quadratic.