Since they want only the coordinates of the vertices. You only care about where the lines intersect, the greater than, less than signs are irrelevant.
Get each equation in 'y=mx+b' form 1) y = -x + 9 2) y = 2x-21 3) y = -4x +15
Now you can set any 2 equations equal to each other and solve for 'x'. This will be the 'x' coordinate of the point where the 2 lines intersect. You need 3 points, so you will need 3 different sets of 2 equations:
1) = 2) 2x -21 = -x+9 3x = 30 x = 10 -----> y = -(10)+9 = -1
1) = 3) -x+9 = -4x+15 3x = 6 x = 2 -------> y = -(2) + 9 = 7
