Find the coordinates of the vertices of the figure formed by each system of inequalities.
18. y + x>-4
y>x-6
3y +x< 10
a. (1, -5). (-14, 8), (-11, 7)
b. (1,5), (7, 1), (11,7)
c. (1,7), (-11, 1), (7,-5)
d. (1,-5), (7, 1), (-11, 7)