<?xml version="1.0" encoding="UTF-8"?>
<math xmlns="http://www.w3.org/1998/Math/MathML">
<mfenced close="}" open="{">
<mrow>
<mn>3</mn>
<mi>k</mi>
<mo>+</mo>
<mn>4</mn>
<mi> </mi>
</mrow>
</mfenced>
<mo>⊂</mo>
<mi mathvariant="fraktur">C</mi>
</math>