The semitone is the smallest interval in music - it is the distance between one note and the next (C to C sharp/D Flat, G to G Flat/F sharp etc)
Hopefully this helps! You didnt provide any notes, so I just demonstrated the rhythm. Good luck on whatever you're doing, if you need any more help let me know in the comments!
You did right by subtracting 12 on both sides but after you subtracted you should have got -192 then you would divide 9 on both sides and once you have found what x equals to substitute what you have for x in your original equation
