Find the missing denominator. \[ \frac{7}{8}=\frac{21}{?} \]

Step 1 ：The problem is asking for the missing denominator in the equation \(\frac{7}{8} = \frac{21}{?}\).

Step 2 ：The two fractions are equivalent, so the cross product of the numerators and denominators should be equal. This means that 7 times the unknown denominator should equal 8 times 21.

Step 3 ：Let's denote the missing denominator as 'x'. So, we have the equation 7*x = 8*21.

Step 4 ：Solving this equation for 'x', we get x = \(\frac{8*21}{7}\).

Step 5 ：Calculating the right side of the equation, we find that x = 24.

Step 6 ：So, the missing denominator is 24.

Step 7 ：Final Answer: The missing denominator is \(\boxed{24}\).