[apps/regression] Use a new helper to compute linear regression coefficients

apps/regression/linear_model_helper.cpp

@@ -0,0 +1,17 @@
+#include "linear_model_helper.h"
+
+namespace Regression {
+
+namespace LinearModelHelper {
+
+double Slope(double covariance, double variance) {
+  return covariance / variance;
+}
+
+double YIntercept(double meanOfY, double meanOfX, double slope) {
+  return meanOfY - slope * meanOfX;
+}
+
+}
+
+}