Match a, b, c into their slots in the formula and crank out both roots mechanically, no need to complete the square from scratch each time.
Module goal. Push the completing-the-square process through once and for all into a universal formula, and use the expression under the root to judge the roots in advance.
eastmath.com · 11.3 The Quadratic Formula and the Discriminant · 11.3.2 Solving by Applying the Formula