Porque tres puntos cualesquiera no determinan un plano

check above link.

graphically?   well, you graph them both, the solutions is where they both intersect each other, since there are two solutions, meaning they will meet twice, or namely the linear will run into the quadratic curve twice, and where they meet, those x,y coordinates are the solutions to the system.

the answer is 40.

proof:

beth buys 10 x shirts and still has more than 100 in her account