1. **State the problem:** Solve the equation $3ra = d^2 + 4$ for $a$. 2. **Formula and rules:** To isolate $a$, divide both sides of the equation by $3r$. Remember, division by a variable is valid as long as that variable is not zero. 3. **Isolate $a$:** $$3ra = d^2 + 4$$ Divide both sides by $3r$: $$\frac{\cancel{3r}a}{\cancel{3r}} = \frac{d^2 + 4}{3r}$$ 4. **Simplify:** $$a = \frac{d^2 + 4}{3r}$$ 5. **Final answer:** $$a = \frac{d^2 + 4}{3r}$$ This means $a$ equals the quantity $d^2 + 4$ divided by $3r$.