--A gallon of enamel paint can cover 300 square feet. Find how many gallon containers of paint should be bought to paint
three coats on a wall measuring 21 feet by 8 feet.

4 Gallons of paint

Step-by-step explanation:

300 square feet & 3 coats measuring 21 ft 8 ft

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.