Are combinations of numbers and operations what is the word that fills in the blank

So the question is blank are compbinations of numbers and operations
the answer is a numberical expression or a closed phrase

open phrase=numbers operations and placeholders exg 3+x
closed phrase=numbers operations no placeholder exg 3+4

open sentence=numbers operations placeholder and relation symbol (=><) exg 3+x=8

closed senctance=number operations no placeholder and relation symbol exg
3+4=9

so it's a closed phrase