Counting differences between strings

Hello!

I have a homework, I have more than one amino acid sequences. I need to count all substitions between these sequences. For example first sequence is 'ALLSPQWGET' second one is 'AKLLQPWTT' and the third one is 'LLLSRQTGEE'. The program needs to compare first and second one and note substitions to corresponding site of the 20x20 matrix. And then it compares first one and second one then second one and third one. I couldn't figure out how to do that. Can you please help me at least giving me some examples or practices to work on?

[Stephen23]

https://www.mathworks.com/matlabcentral/answers/377412-comparing-two-different-strings