Sometimes, you cannot use Extend/Trim tool because these two line will intersect at a virtual point.
Instead of creating another line to extend the existing lines and trim it after, you can use 'Fillet' tool to acheive this. Type 'F' in the command line and select the first line and second line, then you will get an angled line that is extended and trimmed perfectly. Do not forget to change the radius to zero!