Why is it useful to rewrite a formula in terms of another variable?

Sometimes you are given one value and you have to find another value in a formula. For example if you're pouring a concrete sidewalk you'll need to know the length, width, and depth to calculate the volume of the concrete you'll have to buy. Another possibility is someone gives you concrete and you know the volume of it and you need to find out what size of a sidewalk you can make using the given volume of concrete.

22.

Aand b is what i think