Solving linear systems using matrices

Question

needing to find matrix and solve matrix

Maximize: P=20x+8y+12z<=0

Limited budget constraint: 10x+6y+2z<=1800

$capital constraint: 10x+4y+12z<=4000

non negative constraints: x>=0, y>=0

Answer 1

using cramer's rule and removing the inequality signs (just the equal signs)

find the three planes meet at the point x=-1291, y=2230 and z=665

that is the point they meet, note x is not greater than zero

How to proceed I don't know.