Within the body of a template, such as the '''<nowiki>{{Solution | ... }}</nowiki>''' template, the wiki will interpret an '''=''' sign as indicating a named parameter, which will lead to the template not being displayed correctly. Therefore, either avoid the use of the '''=''' sign, or make use the of [[template:=]], like this:
<prenowiki>{{Solution |1 + 1 <nowiki>{{</nowiki> = <nowiki>}}</nowiki> 2}}</prenowiki>
Alternatively, if you wish to use an '''=''' in an equation, you can drop into math mode as in: