When you subtract a negative number, the sign flips and you add instead. Often, you are taught that two similar signs means add and two opposite signs mean minus.
<em>+ + = +</em>
<em>- - = -</em>
<em>+ - = -</em>
<em>- + = -</em>
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This means -3/5 - -2/5 becomes -3/5 + 2/5.
Because you are adding, you move two lines to the right on the number line.