Merge pull request openwebwork#1158 from dlglin/FixSLUQuadratic

Fix vertex formula for quadratic

Author: drdrew42

Contrib/SLU/excelcalculus/ch1/sec2/ch1.sec2.q3.pg

@@ -36,10 +36,10 @@ $x = random(-5,5,1);
 $b = -2*$x;
 
 $popup = PopUp(
-  ["?",'Make -2b/a =$x.',
+  ["?",'Make -b/2a =$x.',
 'Make a=$x','Make b=$x','Make c=$x','Make 2b/a=$x',
 ],
-  'Make -2b/a =$x.',
+  'Make -b/2a =$x.',
 );
 
 BEGIN_TEXT