Use Compute not Formula in answer

Author: gajennings

OpenProblemLibrary/PCC/BasicAlgebra/Calculator/DesmosSolvingRationalEquation10.pg

@@ -73,7 +73,7 @@ $right = Formula("$b")->reduce;
 $leftX = Formula("x/(x+$a)")->reduce;
 $rightX = Formula("$b")->reduce;
 
-$ans = Formula("$var=$x");
+$ans = Compute("$var=$x");
 $y = $left->eval($var=>$x);
 
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