Q: Change numbers in text field to numbers to calculate

I'm making a Quadratic Solver. I want to use the value of a,b,c that the users enter in the text field to calculate x using the ormula x=(-b+-√(b^2-4ac))/2a.

Can anyone show me the way to change numbers in the text field to number can be calculated.

import UIKit

class ViewController: UIViewController {

    @IBOutlet weak var a: UITextField!

    @IBOutlet weak var b: UITextField!

    @IBOutlet weak var c: UITextField!

    @IBAction func solve(_ sender: AnyObject) {

     if a.text == "1"

    {equation.text = "x^2 + \(b.text!) x + \(c.text!) = 0"}

     else

    {equation.text = "\(a.text!) x^2 + \(b.text!) x + \(c.text!) = 0"}

    if a.text==""

    {equation.text="Please enter a"}

    else if b.text==""

    {equation.text="Please enter b"}

    else if c.text==""

    {equation.text="Please enter c"}

                    }

    @IBOutlet weak var equation: UILabel!

    @IBOutlet weak var x1: UILabel!

    @IBOutlet weak var x2: UILabel!

    override func viewDidLoad() {

        super.viewDidLoad()

        // Do any additional setup after loading the view, typically from a nib.

    }

    override func didReceiveMemoryWarning() {

        super.didReceiveMemoryWarning()

        // Dispose of any resources that can be recreated.

    }

}