(x-3)(x-7)-2x(3x-5)

To simplify the expression \((x-3)(x-7)-2x(3x-5)\), we can start by expanding each term.

1. Expand \((x-3)(x-7)\):

   \[ (x-3)(x-7) = x^2 - 7x - 3x + 21 = x^2 - 10x + 21 \]

2. Expand \(-2x(3x-5)\):

   \[ -2x(3x-5) = -2x \cdot 3x + (-2x)(-5) = -6x^2 + 10x \]

3. Combine the results:

   Now, combine both expanded expressions:

   \[ x^2 - 10x + 21 - 6x^2 + 10x \]

   Combine like terms:

   • For \(x^2\): \(x^2 - 6x^2 = -5x^2\)
   • For \(x\): \(-10x + 10x = 0\)
   • Constant: \(21\)

   So, combining everything gives:

   \[ -5x^2 + 21 \]

Thus, the simplified expression is:

\[ \boxed{-5x^2 + 21} \]