XML + String Help

I have two stings that are in XML Format. I want to take one of the strings and combine it into the other string.

so basically i would like to load string1 into an XMLDocument
and take string2, and put that inside the XMLDocument as well.


for example
String1:
<TEST>
<problem1>
<value>4*5</value>
</problem1>
</TEST>

String2:
<solution1>
<answer>20</answer>
<solution1>

what i want is:
<TEST>
<problem1>
<value>4*5</value>
</problem1>
<solution1>
<answer>20</answer>
<solution1>
</TEST>

Any help would be great! I'm using ASP.NET to do this.

Thanks in advance
Greg