linear equations! Options

[pointeitoutx3]

i need to do this and i don't get it ! i've tried like 100 times! help please!

AB y= -2x + 21
BC y= 1x - 12
CA y=2

a) Choose two of the above equations to form a linear system

b) Solve the linear system (substitution method of linear combination method.)

c) Show that the point above is on the third line by substituting the coordinates into the equation.

thanks :) i'm almost pulling my hair out over this.

[Smarmosaur]

i think AB, BC, and CA are the names of the segments (lines) that you're trying to prove.
remember this could be and is most likely COMPLETELY wrong, i'm just doing what i think would make sense with the given information, so don't harass me!
so, the point on y on the graph for line CA would be 2.
and since it already tells you that y=2, substitute 2 in for y in the equations...right?


wait. don't you graph using a slope? or am I thinking of something completely different? o_O.