1. **State the problem:** We are given two equations: $$x = 5y + 9$$ and $$z = 7x + 12$$. We need to substitute the expression for $x$ from the first equation into the second equation to find the resulting equation in terms of $y$. 2. **Write the substitution formula:** Substitution means replacing $x$ in the second equation with the expression from the first equation. 3. **Perform the substitution:** Replace $x$ in $$z = 7x + 12$$ with $$5y + 9$$: $$z = 7(5y + 9) + 12$$ 4. **Simplify the expression:** $$z = 7 \times 5y + 7 \times 9 + 12$$ $$z = 35y + 63 + 12$$ $$z = 35y + 75$$ 5. **Final answer:** The equation after substitution is: $$z = 35y + 75$$ This means $z$ is expressed in terms of $y$ after substituting $x$ from the first equation.