TL;DR Solve a 9x9 matrix without negative solutions.
To make one thing clear! - I have no idea what I'm doing.
What I do know however is to reproduce an equation with different coefficients every time. More specific - this is a game where I'm offered to purchase a set of weapons for a price. I will always get atleast 1 of each weapon, there are 9 types.
So I know how much the total cost is, and how many I get of each - I want to know the individual prices that the swords are sold for.
This is what I think of; where $a$ through $k$ are known.
$ax_1 + bx_2 + cx_3 + dx_4 + ex_5 + fx_6 + gx_7 + hx_8 + jx_9 = k$
I tried using this website at the 9x9 calculator, entered 9 different equations and got values for each $x_i$, $i\in[1,9]$. The problem is that some of the values were negative.
How do I solve this without getting negative values?