book Maths Puzzle Numerical Ability

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

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HI, I do not know if this statement is true or false(why?)

The row space and the nullspace of an invertible matrix are the same

Ax=b has no solution, then b is not in the col what is sqrt36a^2

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