And my answer:
Yes and no.
I find the maximum, you must set any functions that cross equal to each
other.
y1 = y2
x = 9, y = 6
y2 = y3
x = 5, y = 9
y3 = y4
x = 3, y = 6
But also
y1 = 2 (y = 2)
x = 10, y = 2
y4 = 2 (y = 2)
x = 6, y = 2
Usually, the maximum will be the furthest point away from the origin.
(9,6) Z = 4.8
(5,9) Z = 5.5
(3,6) Z = 3.6
(10,2) Z = 3
(6,2) Z = 2.2
(5,9) would be maximize the problem.
And a graph:
http://www.thelouisguy.com/files/probmaxgraph.pdf