Okay so S stands for Seith and B stands for the brother.
You start be writing the equations implied. The first equation is S=2B+3 and the second is S+B=24. You substitute the first equation into the second. Which gives you 2B+3+B=24 adding it together gives you 3B+3=24
3B=21
B=7. So the brother’s age is 7 and when you sub that into the second equation you find that S=17. So the brother’s age is 7 and Seith’s age is 17