CalculateViewController.swift

import UIKit

class CalculateViewController: UIViewController {
    
    @IBOutlet weak var heightLabel: UILabel!
    @IBOutlet weak var weightLabel: UILabel!
    @IBOutlet weak var heightSlider: UISlider!
    @IBOutlet weak var weightSlider: UISlider!
    
  //1. Create bmi property to store the calculated value.
    var bmi: Float = 0.0
    
    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view.
    }

    @IBAction func heightSliderChanged(_ sender: UISlider) {
        let height = String(format: "%.2f", sender.value)
        heightLabel.text = height + "m"
    }
    
    @IBAction func weightSliderChanged(_ sender: UISlider) {
        let weight = String(format: "%.0f", sender.value)
        weightLabel.text = weight + "Kg"
    }
    
    @IBAction func calculatePressed(_ sender: UIButton) {
        let height = heightSlider.value
        let weight = weightSlider.value
      
      //2. Calculate the bmi and store it in the bmi property.
        bmi = weight / (height * height)
        
        performSegue(withIdentifier: "goToResult", sender: self)
    }
    
    override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
        if segue.identifier == "goToResult" {
            let destinationVC = segue.destination as! ResultViewController
          
        //3. Turn the calculated bmi into a String with 1 decimal place, then set it as the value of bmiValue int he destination ResultViewController. 
            destinationVC.bmiValue = String(format: "%.1f", bmi)
        }
    }
}

ResultViewController.swift

import UIKit

class ResultViewController: UIViewController {
    
    var bmiValue: String?

    @IBOutlet weak var bmiLabel: UILabel!
    @IBOutlet weak var adviceLabel: UILabel!
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        //4. Use the bmiValue passed over from the CalculateViewController as the text in the bmiLabel.
        bmiLabel.text = bmiValue
    }

    @IBAction func recalculatePressed(_ sender: UIButton) {
        
    }
}