1. **State the problem:** Given the equation $y=\frac{x}{-3}-1$ and $x=27$, find the value of $y$. 2. **Formula used:** The equation is already given as $y=\frac{x}{-3}-1$. 3. **Substitute the value of $x$:** Replace $x$ with 27 in the equation: $$y=\frac{27}{-3}-1$$ 4. **Simplify the fraction:** $$y=\frac{\cancel{27}}{\cancel{-3}}-1 = -9 - 1$$ 5. **Calculate the final value:** $$y = -9 - 1 = -10$$ 6. **Answer:** When $x=27$, $y=-10$.