📚 Quadratic Equation Solver and ID Validator A Java program that validates user IDs and provides functionalities to solve quadratic equations, find their roots, and combine two equations.

calculate(int a, int b, int c) calculates the root of the quadratic equation ax^2+bx+c=0. If there are two roots, return an array containing the two roots, i.e. new double[] { root1, root2 }.