x5 - 1/2 * x + 7 * x3 - 2x + 1/5 * x3 + 3 * x4 - x5 + 2/5 + 15 = 23,1
=> (x5 - x5) + (7 * x3 + 1/5 * x3) + (-1/2 * x - 2x) + 3 * x4 + 2/5 + 15 = 23,1
=> 0 + (36 * x3) /5 + (-5x)/2 + 3 * x4 + 15,4 = 23,1
=> (36 * x3) /5 + (-5x)/2 + 3 * x4 = 23,1 - 15,4 = 7,7
=> ............