eq:a*x^2+b*x+c=y;
eqSys:[
ev(eq, x = x1, y = y1),
ev(eq, x = x2, y = y2),
ev(eq, x = x3, y = y3)];
s:fullratsimp(solve(eqSys, [a, b, c]));
eq2: diff(eq, x);
r : fullratsimp(solve(ev(eq2, s), x));
string(r);