How do I count the number of cells of a dictionary with n cells which contain the character "a" ?

I have a dictionary containing n cells and I want to know how many of those cells contain the letter "a",

For example- If I have a dictionary with 3 cells which contain the letters a,a,b and I want to find the number of cells in a dictionary which contain the letter "a" which is the value of a variable then the output that I should get should be 2 and along with that I also want to know the Index Numbers of those cells which in this case is X1 and X2,But I am unable to do this,Can anyone help me to achieve this.