Determine the total capacitance of a plate capacitor (high school physics)?

Hello, in physics we're supposed to work on a problem about calculating the total capacitance of a parallel-plate capacitor. We've been given a formula that we're supposed to derive, or rather, explain its composition physically and prove that this equation can be used to calculate the total capacitance. Can anyone help me with this? The task is attached below as an image.

Thanks in advance 🙂