k $\frac{-20 y}{7 x} \times \frac{-21 z}{5 y}$

Step 1 ：The given expression is \(k \frac{-20 y}{7 x} \times \frac{-21 z}{5 y}\)

Step 2 ：First, we can cancel out the common terms in the numerator and the denominator. In this case, 'y' is present in the numerator of the first fraction and the denominator of the second fraction, so it can be cancelled out.

Step 3 ：Next, we multiply the remaining terms in the numerators together and the remaining terms in the denominators together. This gives us \(k \frac{20 \times 21 z}{7 \times 5 x}\)

Step 4 ：Simplifying the fraction, we get \(k \frac{420z}{35x}\)

Step 5 ：Further simplifying, we get \(12kz/x\)

Step 6 ：Final Answer: The simplified form of the given expression is \(\boxed{12kz/x}\)