What is the rule relating to the letter thaal which carries kasrahtain and is followed by laam saakin?

Question
What is the rule relating to the letter thaal which carries kasrahtain and is followed by laam saakin for example 7:8 and 75:12?
pls could you also provide some other examples where this rule is applied?
jazak Allah

Answer
Assalaam alaikum,
When two saakin letters meet between two words, one of the saakin letters needs to change, either by being dropped-as what happens when the first of the two saakin letters is a medd letter, or by acquiring a vowel-which is happens in the case in the question.  It doesn't not need to be a specific letter, any time there is a saakin letter at the end of a word-which is the case with a tanween- and the first pronounced letter of the next word saakin, there is a change that takes place to eliminate the meeting of two saakin letters in pronunciation. 

The rules are laid out for all cases in: http://www.abouttajweed.com/121003.htm

Wa iyyaakum