Consider the region defined by the inequalities: 0≤x

0≤y
-21/4≤y-7/4x
y+x≤7
4≤y-3/7x
Pick up those points which are the boundary points of the resulting convex region.

I would sketch the lines, and by inspection see which lines contained the region, then at each boundry of two lines, analytically figure the common intersection point.