1. The problem is to make the letter in the brackets, which is $a$, the subject of the formula from the equation $k = 4a - b$. 2. Start with the original equation: $$k = 4a - b$$ 3. To isolate $a$, first add $b$ to both sides: $$k + b = 4a - \cancel{b} + b$$ $$k + b = 4a$$ 4. Now divide both sides by 4 to solve for $a$: $$\frac{k + b}{4} = \frac{4a}{4}$$ $$\frac{k + b}{\cancel{4}} = a$$ 5. The subject of the formula is now $a$: $$a = \frac{k + b}{4}$$ This means $a$ is expressed in terms of $k$ and $b$ by dividing the sum of $k$ and $b$ by 4.