The number of arrangements = 9!/(7!2!)=36. If s=7 and t=10:
ssttttttt, ststttttt, sttsttttt, stttstttt, sttttsttt, stttttstt,
sttttttst, sttttttts, tsstttttt, tststtttt, tsttstttt, tstttsttt,
tsttttstt, tstttttst, tstttttts, ttssttttt, ttststttt, ttsttsttt,
ttstttstt, ttsttttst, ttsttttts, tttsstttt, tttststtt, tttsttstt,
tttstttst, tttstttts, ttttssttt, ttttststt, ttttsttst, ttttsttts,
tttttsstt, tttttstst, tttttstts, ttttttsst, ttttttsts, tttttttss.