Program in c# , first before second letter, kindly help me with code

Question

User 3 input string,one letter, second letter then check every instance of first letter occure before every instance of second letter all string should be in lower case ex, " a rabbit jumps joyfully", "a","j" then output should be true,ex," happy birthday","a","y" false

Answer 1

Replace other than 1st and 2nd charecters with empty charecter "". Then find 1st index is less than 2nd for all..