Factor. \[ 3 y-21 \]

Step 1 ：The given expression is \(3y - 21\).

Step 2 ：To factorize it, we need to find the greatest common factor (GCF) of the coefficients of the terms.

Step 3 ：The GCF of 3 and 21 is 3.

Step 4 ：So, we can factorize the expression by taking out the GCF.

Step 5 ：The factored form of the expression \(3y - 21\) is \(3(y - 7)\).

Step 6 ：Final Answer: \(\boxed{3(y - 7)}\)