Hello again, rom!
This is a problem in Linear Programming ... no calculus required.
Part (a)
The units of corn need liters of fertilizer.
The units of wheat need liters of fertilizer.
The fertilizer is limited to 40,500 L.
. . So we have: .
The units of corn need hours to harvest.
The units of wheat need hours to harvest.
The total harvest time is limit to 520 hours.
. . So we have: . Code:

13,500*
*
 *
 *
 *
5250 o *
:::* *
::::::o
:::::::* *
::::::::* *
oo* 
 4500 7000
The graph is the region in Quadrant 1, below the two slanted lines.
. . . . . ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
(b) The profit is 60 per and 40 per
. . . Therefore: .
. . . . . ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
(c) For maximum profit, we consider only the vertices of the shaded region.
We know three vertices: . , and (which we'll disregard).
The intersection of and
. . is: . ... the fourth vertex.
For maximum profit: .
. . . . . ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
(d) The maximum profit is: .