A=lw for l
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Here, you're looking to use your knowledge of inverse operations. That is, multiplication and division are inverses of one another, as are addition and subtraction. In the formula A=l*w, we utilize multiplication. In oder to solve for l, we need to use the inverse of multiplication to "cancel out" the w or "move it to the other side" of the equation. This is the same principle used when solving equations with numbers.
For example, when we ask what x stands for in the following equation, 15=5x, we use our knowledge of inverses to divide both sides of the equation by 5, thus giving 15/5=5x/5, or 3=x. The same principle is used in the problem your proposed, there just aren't any numbers to make it obvious!