for question number 15...cordinated grid shows each equation...first equation is represented by straight line going upwards through +4 in y axis...(y=3x+4)...
point is....every point in this two lines represent a possible x and y value for each equation...and there is a point where these lines meet each other....in that point...x and y values are possible values for both equations.so answer is (-2,-2)...this cordination satisfy each pairs of equation...you can try using -2 for x in both equations and get -2 as the answer for y...that proves the point..try it for 16 quiz.good luck
