1. **Problem:** Make $a$ the subject of the formula $2a + b = 10$. 2. **Formula and rules:** To isolate $a$, subtract $b$ from both sides, then divide both sides by 2. 3. **Step-by-step solution:** $$2a + b = 10$$ Subtract $b$ from both sides: $$2a + \cancel{b} - b = 10 - b \implies 2a = 10 - b$$ Divide both sides by 2: $$\frac{2a}{\cancel{2}} = \frac{10 - b}{2} \implies a = \frac{10 - b}{2}$$ 4. **Explanation:** We moved $b$ to the right side by subtracting it, then divided the entire equation by 2 to solve for $a$. **Final answer:** $$a = \frac{10 - b}{2}$$