Making subject formulas

The subject of a formula is the variable that appears by itself on one side of the equation.

In $A=l\times w$, the variable $A$ is alone on the left-hand side. The right-hand side contains $l\times w$, which are the other variables.

Since $A$ is isolated on its own side of the equation, $A$ is the subject of the formula.

We need to find which variable stands alone on one side of the equals sign in $A=l\times w$.

$l$ appears on the right-hand side multiplied by $w$, so it is not isolated. $l$ is not the subject.

Similarly, $w$ is on the right-hand side multiplied by $l$, so it is not alone. $w$ is not the subject.

Every formula has a subject — the variable written alone on one side. Saying 'there is no subject' is incorrect.

$A$ is the only variable appearing by itself, so $A$ is the subject.