void solve2(Matrix4 A, Vector2 x, Vector2 b)

Solve A * x = b.

Source 

static void solve2(Matrix4 A, Vector2 x, Vector2 b) {
  final double a11 = A.entry(0, 0);
  final double a12 = A.entry(0, 1);
  final double a21 = A.entry(1, 0);
  final double a22 = A.entry(1, 1);
  final double bx = b.x - A._m4storage[8];
  final double by = b.y - A._m4storage[9];
  double det = a11 * a22 - a12 * a21;

  if (det != 0.0) {
    det = 1.0 / det;
  }

  x.x = det * (a22 * bx - a12 * by);
  x.y = det * (a11 * by - a21 * bx);
}