There is only one time you want to return and that is when growPlant is greater than or equal to the desiredHeight.

You need to both add and subtract to/from the growPlant value. Each “day” is one loop iteration, for each loop iteration you both grow and shrink the plant (increment and decrement the growPlant variable by the upSpeed and downSpeed). If at the start of a loop iteration growPlant has the desiredHeight you return how many times the loop ran, i.e. how many “days” it took.

Sometimes you just have to take a break and walk away. Then, when you least expect it, you will think of something that gets you closer, or all the way to the solution. Get familiar with that feeling of believing that you will not find the solution. Accept that feeling as one of the steps toward finding the solution. Your brain will often prove your emotions wrong. But it does take some perseverance. Keep at it!