Append string to another string after a specified character

Append string to another string after a specified character

I have a string a="<tr>i'm working in C#<tr/>", and
b="<td>Finish</td></tr>". I need string a as
a="<tr>i'm working in C#<tr/> <td>Finish</td></tr>".



How do we append string in string like above?