Answer:
a. 15
Step-by-step explanation:
Step 1 - Input the value of 'a' in the expression.
21 - 2a
21 - 2(3)
Step 2 - Multiply two and three
21 - 2(3)
21 - 6
Step 3 - Subtract six from twenty one
21 - 6
15
Therefore, the value of the expression 21 - 2a if a = 3 is a. 15.