What is the equation of a line with slope of -3 that passes through the point (5,-8)

Equation of a line:

y = mx + c

Slope of -3:

m =  -3

y = -3x + c

Passes through the point (5,-8)

x = 5

y = 8

8 = -3(5) + c

8 = -15 + c

Solve to find c:

Add 15 to both sides.

8 + 15 = -15 + 15 + c

8 + 15 = c

23 = c

Equation

y = -3x + 23

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.