cambridge... kinda confused (1 Viewer)

[helpfulperson?]

I always see 'Use box []' to prove something. What does that exactly mean?
Here's an example:

 

[idkkdi]

I always see 'Use box []' to prove something. What does that exactly mean?
Here's an example:View attachment 39482

is it in the chapter

 

[helpfulperson?]

thx. could I ask where you got that solution haha, I only have worked solutions for development and foundation