Design a GUI to calculate capacitance

I need to design a GUI that allows the user to select the resistance value from a drop down menu with 4 choices, which are 1k, 2k, 3k, and 4k. Then the user is required to input the cutoff frequency and the GUI should display the capacitance value.

I want to program it such that when I click a button 'CALCULATE', the program will take the selection from the drop down menu such that if index value is 1, the resistance is set to be 1000 and etc. Then it takes the frequency value and calculate the capacitance using the formula f = 1/2πRC. How should I do it?

The GUI that I have designed looks like this

[Walter Roberson]

Okay, what is your question?

Also, are you using GUIDE or App Designer ?

[Ye Ken Kok]

This is the question given to me

And I am using App Designer