Many students get confused between anything in algebra involving's.

Anything without an equals sign is an expression.

are all expressions. The aim is always to simplify expressions.

Anything with an equals sign containing x's and no other letters, or containing more than two letters with no letter being the subject is an equation.

are all equations. The aim is always to solve equations, if possible.

Anything with more than two letters, with one letter being the subject is a formula. Having a formula means than a quantity can be calculated by substitution.

are formulae, but

is not a formula because neithernoris the subject.

is not a formula forbecauseoccurs on both sides.