Tiny triangle in upper right hand corner of button won’t work.

I am using both the app and online version of Kahn academy on my iPhone. I am trying to enter in the answer to a math problem which is 3(3a-6b+7c). The issue is that when I go to enter in the “b” for the answer the only letter or variable available on the keypad is the letter “a” with a tiny triangle in the upper right hand corner of the button. I am assuming that I am supposed to be able to tap the tiny triangle in the upper right hand corner of the letter/variable button to be able to enter another variable/letter. Unfortunately, I am unable to do so. The button with the tiny triangle in the upper right hand corner will not activate and it only activates the buttons around it. I wanted to know if anybody else has had a similar problem or if a body has an idea on how to work around this problem.