The difference between twice a number and three is equal to six decreased by the number find the number

you can't solve x because it is 2 linear equation and you only have one equation

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.