This is just a tedious problem with no simple approach.

To solve it you must consider all possible cases.

The first case is with no letter repeated: (5!)

Then you might consider “words” with 2 or 3 S’s but on other repeats.

Double that to account for similar cases using T’s.

The another case would be: 2 S’s , 2 T’s and another.

As you can see it is messy to find all cases.

Good Luck.