Solve Both Ways Up

I came across this little puzzle on Twitter, which is rather fun:

Thought I'd share this little algebraic curiosity that I was given by Tim Rowett yesterday, on the back of a business card. Solve for x…both ways up. pic.twitter.com/vGaBtu0LAU

— Rob Eastaway (@robeastaway) December 16, 2018

I was rather pleased that my answers were the same as other people’s: 21 and =/-3.