He gets ax commission for selling x cars where a is a constant. His basic pay is p so he earns y=ax+p.
Now we put in x=11 and y=6300: 6300=11a+p;
and we put in x=24 and y=12800: 12800=24a+p.
Subtract the first equation from the second: 6500=13a, so a=6500/13=500.
So 6300=5500+p, making p=800. Therefore y=500x+800.