How can you rearrange formulas to highlight a quantity of interest, using the same reasoning as in solving equations?

Its probably easier to explain by doing an example.

Make a the subject of the formula

b^2 = 2as + c

subtract c from both sides:-

b^2 - c = 2as

Now divide both sides by 2s (to isolate a) : -

a = (b^2 - c) / 2s