Solving purely symbolic systems of linear equations

Hello,

I'm working through a finite element text which provides a symbolic solution to a system of linear equations which I suspect might be incorrect based on some checks that I've done. I'm interested in double checking their math, but I'm finding it excessively complicated with the techniques that I know how to use. Here is the system:

$\displaystyle u_i=\alpha_1+\alpha_2 x_i + \alpha_3 y_i$

$\displaystyle u_j=\alpha_1+\alpha_2 x_j + \alpha_3 y_j$

$\displaystyle u_m=\alpha_1+\alpha_2 x_m + \alpha_3 y_m$

I'm trying to solve for $\displaystyle \alpha_1, \alpha_2, \alpha_3$.

First I tried substitution by solving the first equation for $\displaystyle \alpha_1$ and plugging into the second equation. Then I tried solving that for $\displaystyle \alpha_2$, etc. The math just got so messy that I gave up.

I next tried setting up the equations in matrix format:

$\displaystyle

\left\[\begin{matrix}1&x_i&y_i\\1&x_j&y_j\\1&x_m&y_m\end{ matrix}\right\]

$