To evaluate the expression \( 9y - 10z \) when \( y = 9 \) and \( z = 6 \), we first substitute the values of \( y \) and \( z \):
-
Substitute \( y = 9 \): \[ 9y = 9 \cdot 9 = 81 \]
-
Substitute \( z = 6 \): \[ 10z = 10 \cdot 6 = 60 \]
Now, replace \( 9y \) and \( 10z \) in the expression: \[ 9y - 10z = 81 - 60 \]
Finally, perform the subtraction: \[ 81 - 60 = 21 \]
Thus, the value of the expression \( 9y - 10z \) is \( \boxed{21} \).