Added add and subtract methods

src/main/TestScientificCalculator

@@ -0,0 +1,44 @@
+package com.zipcodewilmington.scientificcalculator;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class TestScientificCalculator {
+
+    @Test
+    void newCalculatorShouldStartAtZero() {
+        ScientificCalculator calculator = new ScientificCalculator();
+
+        assertEquals(0, calculator.getDisplay());
+    }
+
+    @Test
+    void setDisplayShouldChangeDisplay() {
+        ScientificCalculator calculator = new ScientificCalculator();
+
+        calculator.setDisplay(25);
+
+        assertEquals(25, calculator.getDisplay());
+    }
+
+    @Test
+    void clearShouldResetDisplayToZero() {
+        ScientificCalculator calculator = new ScientificCalculator();
+
+        calculator.setDisplay(25);
+        calculator.clear();
+
+        assertEquals(0, calculator.getDisplay());
+    }
+
+    @Test
+    void addShouldUpdateDisplay() {
+        ScientificCalculator calculator = new ScientificCalculator();
+
+        calculator.setDisplay(5);
+        calculator.add(3);
+
+        assertEquals(8, calculator.getDisplay());
+    }
+}

src/main/java/com/zipcodewilmington/scientificcalculator/ScientificCalculator.java

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+package com.zipcodewilmington.scientificcalculator;
+
+public class ScientificCalculator {
+
+    private double display;
+    private double memory;
+    private boolean error;
+    private String displayMode;
+    private String unitsMode;
+
+   public ScientificCalculator() { 
+   display = 0;
+   memory = 0;
+   error = false;
+   displayMode = "decimal";
+   unitsMode = "degrees";
+   }
+
+   public double getDisplay() {
+    return display;
+   }
+   public void setDisplay(double value) {
+    display = value;   }
+   public void clear() {
+    display = 0;
+    error = false;
+   }
+   public void add(double value) {
+    display = display + value;
+   }
+    public void subtract(double value) {
+    display = display - value;
+}
+
+}