To simplify the expression -\frac{1}{3}(x-3)(x-21), we first distribute -\frac{1}{3} to both terms inside the parentheses. This involves multiplying -\frac{1}{3} by each term in the product of (x-3) and (x-21). After distributing, we combine like terms and simplify further. The final result is -\frac{1}{3}x^2 + 8x - 21, which is a quadratic expression in standard form.