An Odd Error

Using the source code found at http://naveedurrehman.com/demo/phpequations/?page=demo#output, I created a basic PHP file designed to solve a system of linear equations. However, the file returned the following:
Error :) Array ( [0] => Number of equations (4) are not equal to number of variables (8) [1] => Can not load equations [2] => Can not solve )
The system has four equations, and it has four variables (or at least I thought it did). This is the code for solving the system:

$solution = $obj->solve("

a+b+2*c=5
a-b=u
u-2*a=4
2*b=a

");