Depends on calculator. Switch off and on first to clear part calculations.
First enter 15 then multiply (X) then press sin key. If display shows sin(, then type in 31 then =. The answer should be displayed. The answer should be 7.725571124 or something like it.
If this doesn't work, try entering 31 then press sin. The display should show 0.5150380749, or something like it. Now press multiply (X) and enter 15 then =. You should get the answer 7.72...