Work Step by Step
In order to subtract by changing subtraction to addition, we can follow the steps below. Step 1. The first number stays the same. Step 2. Change the subtraction sign to an addition sign. Step 3. Change the second number to its opposite. Step 4. Evaluate. Therefore, $-2-(-3.9)=-2+(3.9)=-2+3.9=3.9-2=1.9$.