implement subtract static and instance method

src/main/java/Rational/Rational.java

@@ -51,17 +51,20 @@ public class Rational {
         return r1.add(r2);
     }
 
-    // TODO: Instance method - this - other
-    // Formula: a/b - c/d = (a*d - c*b) / (b*d)
+    // COMPLETED - Instance method - this - other
     public Rational subtract(Rational other) {
-        // YOUR CODE HERE
-        return null; // Remove this line when implemented
+        int newNumerator = (this.numerator * other.denominator - other.denominator * this.denominator);
+        int newDenominator = this.denominator * other.denominator;
+
+        Rational result = new Rational(newNumerator, newDenominator);
+        result.simplify();
+
+        return result;
     }
 
-    // TODO: Static method - r1 - r2
+    // COMPLETED - Static method - r1 - r2
     public static Rational subtract(Rational r1, Rational r2) {
-        // YOUR CODE HERE
-        return null; // Remove this line when implemented
+        return r1.subtract(r2);
     }
 
     // TODO: Instance method - this * other