(a, b) * ((1, 0), (0, -1)) = ((a * 1 + a * 0), (b * 1 + b * 0))

public double[] multiply (double[] vector, double[][] matrix) {

double[] result = new double[vector.length];

for (int i = 0; i < vector.length; i++) {
for (int j = 0; j < matrix[0].length; j++) {
result[i] += result[i] * matrix[i][j];
}
}

return result;
}