<pre> formatting for index.md (#26986)

Examples were jumbling together, added formatting to help with readability

guide/english/mathematics/adding-and-subtracting-negative-numbers/index.md

@@ -7,15 +7,15 @@ Adding and subtracting positive numbers is fairly straighforward, but what does
 
 ### Examples
 
-(-1) + 3 = 2
+<pre>(-1) + 3 = 2</pre>
 
-(-3) + (-3) = -6
+<pre>(-3) + (-3) = -6</pre>
 
-(-2) - 4 = -6
+<pre>(-2) - 4 = -6</pre>
 
-(-10) + 2 = -8
+<pre>(-10) + 2 = -8</pre>
 
-20 + (-44) = -24
+<pre>20 + (-44) = -24</pre>
 
 #### More Information:
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