book Maths Puzzle Numerical Ability

can you show me your method of doing this

-4x-12y-11z=-53
3x-5y-8z=-9
-9x-10y+7z=-24

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book Other Question

please help me answer this question immediately. how the set of whole numbers and the set of intergers are similar and how they are different? thanks I need to find out (for economics) how to graph the following:
Px = (400/3) + (1/3)Qs
This is for supply and demand. Isn't it true that to graph you set p=0 then solve for q? And